How to make the undergravel filter

by Dave Keenan, 27-Nov-2005


Please steal these ideas and methods, and hopefully improve on them. Even make money out of them if you can. Iíd love to hear what you do with them. See the swimming-ponds Yahoo group: Iíve put significant time, money and effort into researching and testing them (and earlier dead-ends), while supported by my wife Janelle Briggs, so it would be nice if you would acknowledge our contribution. Thanks.

Itís basically a thousand times scale-up (by water volume) of a standard aquarium undergravel filter. The origins of the recognition of the benefits of gravel beds (at first natural ones), for water treatment, are lost in antiquity. One civil engineer told me that the Koran states that when water has passed over three stones it is clean, although I havenít been able to confirm this. My grateful thanks go to Robert P.L. Straughan and others who popularised undergravel filters for home aquaria starting in the 1950s, and the school teachers who let me look after the school's aquaria as a student in the early 70s, and Bill Mollison who suggested applying undergravel filters to swimming ponds in "Permaculture - A designers manual", 1988 (p 178).

Materials lists and instructions are given below, for constructing an undergravel filter for a 30 kilolitre oval aboveground pool (9.6 m x 4.5 m) having two pumps/risers/UV-heads connected to two independent undergravel intake manifolds. It includes materials and instructions for the in-pond mesh-fence that separates the swimming area from the fish and plant refuge. Materials lists and instructions for the pumps, UV-heads and other components are given in separate documents.

Once the basic system is understood you should be able to design systems for other shapes and sizes of pond and calculate the quantities of materials required. I recommend one pump with independent intake manifold for every 10 to 15 kilolitres. To ensure even flow, intake manifolds should be, roughly-speaking, square or circular (or equilateral triangular), which is only to say that they should not be long and thin. They can be any convenient shapes that together fill the available bottom area, but none should have its longest dimension more than about twice its smallest.

I describe an above-ground pool conversion, however it should be clear which steps can be omitted for in-ground pools or pools that are not plastic lined.

You should not try to construct gravel filters on sloping bottoms unless the slope is less than about 1 in 10, otherwise gravel will tend to migrate down the slope over time.

This system does not make use of any existing pumps, pipework, skimmer boxes, water-return eyes or bottom drains, however their integrity may be important for avoiding leaks and accomodating overflows.


Landscape supplier only:

Hardware or building supplies normally have these, but the landscape supplier may also have them:

Before buying this, check that there are no sharp protrusions e.g. mold flashing. Some brands are better than others.

Hardware or building supplies only:

These pots have large holes around their sides that fish can swim through.






The "Before" photo

Lay out the ag-pipe

Some brands of ag-pipe are full of the hundreds of little pieces of plastic punched out of the drainage slots. They rattle when you shake the roll (If that reminds you of a song title, youíre probably as old as I am). You need to remove them. The most effective way I've found might well be called "shake, rattle and roll". You shake the roll up and down while at the same time rotating it, always in the same direction, so that eventually all the little pieces come out of the outermost end of the pipe and it stops rattling.

Unroll the ag-pipe and lay it out straight in the sun, to try to reduce the curl which is a nuisance when constructing the undergravel filter.

Drain the pool

If possible, save the water, e.g. in a rainwater tank or another pond or dam, for later gravel washing and/or refilling. When the level gets too low for pumping or siphoning, use two opposed ice-cream buckets, or an ice cream bucket and lid, to ladle into a 10 litre bucket which you periodically empty. It is more comfortable to work in the pool when itís down to a few small puddles, but it isnít necessary for it to be completely dry or devoid of leaves. It is however essential to remove all hard material such as sticks and stones. This is to prevent possible puncture of the pool liner when the weight of the under-gravel-filter and people are pressing on it.

Find and patch any existing holes or weak spots now. Remember, any holes in the bottom after the gravel is in place, are there forever.

The existing pump and filter can be removed and the skimmer box can be coupled directly to the return eye to effectively close them both off. But you need to think about where the water will go when your pond fills to overflowing with rain. The lid of the skimmer box will often be the lowest point. You donít want water flows undermining your pond, so you may need to dig a drain.

Install the bottom protector

This only applies to ponds with a plastic membrane liner.

Cover the bottom of the pool and at least 100 mm up the sides with a layer of builders plastic. Any old overlapped scraps or offcuts of soft plastic will do, even old poly-tarps with their metal eyelets removed. Just ensure there is complete coverage.

Take care when walking on this layer as it will tend to slide easily where there is water under it - another reason to get the pool bottom as dry as possible before starting. The photo shows the side protector in place at this time, please ignore this, it was a mistake.

Cut and mark the Bidim geotextile

If you have enough helpers, this can be done at the same time as the building of the undergravel manifolds, see below.

You might think, "Since weíve got builderís plastic on the bottom to protect the liner, why do we need the Bidim?". It turns out it is necessary to stop the ag-pipes from "floating". The ag-pipes with water inside them are less dense than the gravel and water outside them, and over time, with people walking over them, gravel would be forced under them and they would eventually "float" to the top of the gravel and then pop right out of it. So the gravel holds the bidim down which holds the pipes down which hold the mesh down. But we make the Bidim serve other purposes too. We make it continuously overlapped so that it does provide more protection of the bottom liner from possible sharp gravel pieces. It also provides lots of surface area for good bacteria to colonise, in fact I sometimes wonder if most of the work is done by bacteria on the Bidim rather than the gravel.

Sweep an area of concrete driveway, lawn or verandah completely clear of sticks and stones, on which to unroll, cut and mark the bidim. Small sharp or hard objects easily become entangled in the fibres of the bidim and if transferred to the pond, may puncture the liner.

Bidim widths need to be overlapped by at least 100 mm to cover the whole bottom of the pond and to go 300 mm up the walls. In the direction at right angles to the PVC trunks of the undergravel manifold we need to add 100 mm for each trunk it must wrap over. In the direction at right angles to the ag-pipe branches of the manifold the bidim must be made long enough to wrap most of the way around each ag-pipe. For 50 mm ag-pipe this means adding 140 mm for each ag-pipe crossed.

The oval pool of this example is 4.5 m x 9.6 m. It will have two trunks with 19 branches on each side (230 mm spacing). So in the long dimension we need 9.6 + 2x0.3 + 2x0.1 = 10.4 m. The bidim comes in a 2 m width, but with 100 mm overlaps we can only count on 1.9 m. So we need 10.4/1.9 = 5.47 widths. We can get away with only 5 widths because the corner off-cuts, due to it being an oval, will be sufficient to finish the ends. In the short dimension we need 4.5 + 2x0.3 + 19x0.14 = 7.76 m. So we cut five 7.76 m lengths of bidim.

It is nearly impossible to get the bidim evenly arranged between all the ag-pipes without marking it, so before we put it into the pond we measure and mark where the top centre of every ag-pipe will be, on the bidim, with a thick permanent marker. We mark each width of bidim on both edges and in the middle. The spacing between the marks must be 140 mm wider than the spacing of the ag-pipe centres, in this case 230 + 140 = 370 mm apart. It should only be necessary to measure and mark one length. You can then use this as a template to mark the other lengths, but do thoroughly check your measurements first. You might fold it in half to find the centre and measure from the centre out. Put a special mark such as a "C" on the center mark and when fully marked roll them up (from each end to the middle) ready to install in the pond after the under-gravel manifold is in place.

Make the side-wall protector

If you have enough helpers, this can be done at the same time as the building of the undergravel manifolds, see below.

This isnít to hold water, but is installed around the walls as a full length "skirt" whose bottom edge is just under the gravel. This is to cover the existing liner (usually pale blue) to (a) make it a more natural colour and (b) protect it from the gravel. It isnít required for in-ground pools that donít have a plastic membrane liner Ė these should be painted with black pool paint. If you used ordinary builders plastic (200 um polyethylene) it would go brittle and crack from UV exposure after about six months, where it is exposed to direct sun above the waterline. You should instead use a UV-stabilised black pond liner (e.g. 500 um UV-stabilised black PVC) but itís about 10 times the price. If youíre really on a tight budget you could use the builders plastic and add a "mini-skirt" of pond-liner over that, which covers only the area above the low water mark and at least 100 mm below it. See

Apparently it isnít the water itself that protects the plastic below the water line from UV, but rather the film of algae and other microorganisms that grows there. So if you plan to keep it very clean you should use UV-stabilised material all the way down. And of course this is much simpler and gives a better looking result. It just costs a lot more.

The following describes the more complicated but lower-cost alternative using builders plastic. If using all UV-stabilised material then use the joining method recommended by the manufacturer for the material you choose.

On a clear area of concrete driveway, lawn or verandah, unroll the 9 m of black plastic, then unfold it to 4 m wide. Try not to walk, kneel or lean on it except when the smooth board is underneath.

We need to cut it into three equal strips 9 m x 1.333 m. To mark it for cutting, measure 1333 mm in from the long edges and mark with slivers of grey duct-tape at about 1 m intervals. Cut with scissors, sighting between the slivers of duct tape. Alternatively, just fold it in three and cut on the folds.

Now we need to tape these three strips together to make one long strip about 27 m x 1.333 m. Overlap the ends of two pieces by 50 to 100 mm. They must be dry and free of dust or oil. You may need to partially roll them up to do this. Use the smooth board underneath the join. Tape the join with duct tape, but please note that it is extremely important to allow time for the duct tape to return to its normal size after peeling it off the roll and before sticking it down. Hold the tape slackly and count to at least 30. Otherwise it will continue to contract after sticking down and will cause the joint to buckle and gather and will be more likely to separate from the plastic again underwater.

Turn the two sheets of plastic over and run a fat continuous bead of Selleys "All Clear" copolymer sealant between them and immediately press them together hard, before the sealant has a chance to form a skin. Then tape this side of the join (remember to relax the tape). Repeat the process to join on the third piece.

Donít bother trying sealants other than "All Clear", such as silicones. Nothing else sticks to polyethylene. Selleys donít even claim that "All Clear" will stick to polyethylene, and it is extremely slow curing (months), hence the duct tape. Donít be tempted to use duct tape alone. Although it is wonderful stuff, its adhesive wonít last more than a year in biologically active water.

Now you need to do the same for the UV stabilised mini-skirt. That is, cut and join it to give the required width and length. You could use proper PVC pond-liner adhesive here (or whatever the manufacturer recommends) but All Clear works well here too. A typical width for it is 500 mm, so you can cut four from a 2 m width.

An aside: You can actually make a cheap waterproof pond liner (the part that actually holds the water), out of builders plastic by the above duct-tape and "All Clear" method. Use only the thicker 200 um builders plastic in the 4 m widths. You can join two 4 m widths in this manner to make an (almost) 8 m width. The usual disclaimers about UV apply, so you need the protective "mini-skirt" above the low waterline and at least 100 mm below it.

This side-protector skirt and UV-protection mini-skirt must be installed after the undergravel filter manifold and bidim (geotextile), but before the gravel. Youíll see that we didnít do this for the pond in the photos and it was a nuisance having to repeatedly lift it up while assembling the manifold and Bidim. Nor did we install the UV protection mini-skirt. This was corrected much later, with enormous difficulty. Live and learn.

Make the under-gravel manifolds

These are composed of 50 mm ag-pipe "branches" coming out both sides of a "trunk" consisting of 100 x 50 mm PVC downpipe laid flat.

First determine how many pumps/risers and therefore how many independent manifolds are required. You need one for every 10 to 15 kilolitres of pond volume. Use the one per 10 kL figure if the pond gets a lot of sun. There is some reason to think that a calculation based on square metres of surface may be more valid than one based on volume, although I suspect the truth lies somewhere in between. The above assumes a depth of about 1.0 to 1.2 metres. If your pond is significantly deeper than that you should use the one per 15 kL figure. Try to arrange for their collection areas to be roughly equal in size and roughly square to circular.

Consider where you will place the mesh fence for the "fish and plant refuge". This refuge would ideally occupy about one third of the area of the pond. The refuge you see in the photo is really too narrow to support a sufficient number of plants. It should be at least one sixth of the area of the pond. In the 30 kL oval pond under consideration it should be a strip about 1 m wide along the long side that gets the most sun, e.g. the southern side in the southern hemisphere. It may be convenient to tie the refugeís end fence posts to the posts on the outside of an aboveground pool, so the position of these may determine the exact placement.

It is important to site the risers (and hence one end of each trunk) inside the refuge. For oval ponds, trunks at right angles to the straight or long sides make sense. In circular ponds, radial trunks may make sense, in which case you may have to have a separate refuge associated with each riser.

In the case of an in-ground pool the ideal is to build a mini-wetland right beside your pool by plastic-lining a hole about 400 mm deep and of about half the area of the pool, with its water surface at the level of the pool coping. The pumps will then feed water from the undergravel filter into this wetland and past all the plants (for nutrient removal) before it returns to the swimming area. In the case of an above-ground pool should try to approximate that ideal as best we can, completely within the pool,.

You might be tempted to make a wetland beside the pool even when the two bodies of water cannot be at the same level. Please forget about this, unless they can be within about 200 mm of each otherís levels. Otherwise, the energy required to lift the water will completely negate the saving made by using the in-pond biofilter in the first place, and you will not be able to use an axial pump. For completely in-pond circulation with our DIY axial pumps, we only consume energy at the rate of about 6 watts (joules per second) for every litre per second of flow (you should aim to turn over your pond volume every 2 hours). For every metre of lift you add to this, energy consumption will go up by at least 30 watts (for each litre-per-second of flow), possibly much more.

The pumping equation is P = mgh/e, where P is power in watts, m is mass flow rate in kilograms per second (same as litres per second in the case of water), g is gravity (around 10 if youíre on planet Earth), h is height in metres, e is efficiency (typically 0.3 to 0.4 with an ideally matched pump).

Once youíve divided the area of the pond up into enough approximately-equal collection areas, then you need to decide where to put the trunk within each collection area so it has about half the collection area on either side of it. Draw yourself a diagram to scale and put some measurements on it. Think about how the plastic mesh will be laid horizontally on top of the manifold and tied down to it (this mesh is the "boy-proofing" in the gravel, not the vertical fence for the refuge).

The plastic mesh I used was 920 mm wide, allowing the minimum necessary overlap for joining (check yours, I think they have changed the design of this stuff since I first bought it), so a quarter of this (230 mm) is a convenient spacing for the ag pipe centers. Donít make them any further apart than 230 mm. If you donít need to match them to mesh joins it would be better to place them closer, say at 200 mm centers (four times the ag-pipe diameter).

Measure the length required for the trunks at the flat part of the pool bottom only. i.e. Stop them before the curved "meniscus" or fillet that comes up to meet the sides. This is so that the corners of the trunk will not dig into the liner. Cut the 50x100 mm downpipe 80 mm longer than required. Join lengths with standard joiners and PVC cement if necessary, working outside the pond.

You can make your own joiner using an offcut about 100 mm long, although it wonít be anywhere near as strong as a bought joiner. Cut the offcut along the center of one 100 mm face so it can open up to wrap around the two pipes to be joined. Heat its bends with the flame from a pipe-lighter or mini gas-torch and reform them so it conforms properly to the outside of the downpipes. Glue it to the two pipes with gap-filling PVC cement and use duct tape to cover the remaining gap and hold the ends together while it sets (preferably overnight). Leave the duct tape in place permanently and make sure it faces down when installed in the pond.

Now we close the ends off. Working outside the pond, mark a line all around the trunk 40 mm from each end. Cut each corner in to the line with a hacksaw. Heat along the line with the flame from a pipe-lighter or mini gas-torch and bend each flap by 90 degrees and hold it until it cools, so as to close off the ends of the trunk. Round off the corners with a file to reduce the chance of liner damage. Fill any remaining holes or gaps with a silicone sealant.

Now we cut the holes for the branches. Mark the center of one 50 mm side of the trunk. This will be the center of one 48 mm hole into which an ag-pipe "branch" will be inserted. Mark the centers of the other holes at intervals of 230 mm either side of this until you are less than 375 mm from an end. Then mark hole centers 30 mm in from each end. Be sure all marks are accurately centered within the 50 mm width. Do the same on the opposite 50 mm-wide side of the trunk. Use a 48 mm holesaw to cut all these holes. Go slowly so as not to tear the thin PVC. Remove all cut out disks and swarf. Clean up any jagged edges that might damage the liner. Minor accidents may be repairable with sealant and duct tape once the branches are in place.

Now we cut the hole for the pump intake. The pump will plug into a 100 mm straight pipe-joiner attached to the top of the undergravel filter trunk. Take the 100 mm PVC straight pipe-joiner and place it against the top face of the trunk, centered on the trunk and as close as practical to one end of the trunk, without overhanging the end. Draw the circle inside this with a marker pen. The pipe joiner will be slightly wider than the top face of the trunk and we donít want to cut into the sides of the trunk so we end up with a circle having two flat sides. Use a jigsaw to cut this out of the trunk. The photo shows a different (older) arrangement for the pump intake. Please ignore this.

Glue the pipe joiner onto the trunk with gap-filling PVC solvent cement, but donít worry about the large gaps on the two flat sides for now. When this has had about half an hour to set, build up a fillet of PVC cement around the outside of the join (for greater strength) and fill the gaps on the two flat sides with silicone sealant. Leave it to set, preferably overnight. That completes the trunk.

When the pond is in operation, whenever you remove the pump you should immediately plug the hole to prevent fish swimming into it and being trapped there when the pump is replaced. Fish just love caves like this and would get mangled in the propeller pump when they finally decided to come out. A convenient plug is a length of 100 mm PVC pipe that extends above the water surface, or a spare pump.

Mark Roberts (Aussie swimming pond number three) pointed out that you can get 100 mm sewer-pipe inspection caps which can also function as pipe joiners, and that way you have a cap you can screw on when the pump is removed.

Ag-pipe branches

Cut a piece of bidim slightly wider and longer than the trunk. Lay the completed trunk on top of it, in its correct position on the bottom of the pond. The bidim is to help protect the liner from any remaining sharp points or edges on the trunk, although these should have been filed as smooth as possible.

The pump intakes should be on the side of the pond that will get the most sunlight, i.e. the south side if possible, in the southern hemisphere. This is because the risers should be inside the fish and plant refuge, and plants need sun.

Measure the required lengths of all the branches (the lengths of ag-pipe that will go into the holes along the sides of the trunk) and make up a cutting list. Alternatively take the measurements off a scale drawing (but do check them in the pond). As with the trunk, only run them up to the start of the upward curve near the walls. The branches will be inserted into the holes in the trunk only two "clicks", that is two ridges of their corrugations.

Cut the lengths of 50 mm ag-pipe from the cutting list using a hacksaw in the groove between two corrugations.

Close one end of each piece with duct tape. Use three pieces of tape about 200 mm long. Wrap two of them over the end of the pipe, at right angles to each other. Wrap the third piece around the pipe, projecting past the end of the pipe by 5 to 10 mm. This third piece wraps over the ends of the other two pieces to help hold them in place and when the projecting part is pressed down over the end it covers any small holes that may have been left by the other two pieces. You can buy end caps for ag-pipe but I have found their cost to be typically more than that of all the ag-pipe, and even when the duct-tape adhesive lets go it will still be held in place by the gravel.

Force the branches into the holes in the trunk. Push only two corrugations of the ag-pipe into the trunk. These joints do not need to be sealed as they are tight enough to be comparable to the slots that are in the ag-pipe anyway.

Rotate each ag-pipe branch so that the curl from the roll tends to make it arch upwards in the middle. That way they will tend to lie straight. But ensure that any sharp edges due to mold flashing are not pointing downwards. Arrange them so they are parallel and evenly spaced. This photo, and several following, show the side liner protector in place, but in fact it should not have been installed until after the Bidim and Hill clips were installed.

Make the Hill clips

Itís especially good to have a few friends or family (including kids) to help with this job because although it is very easy, it is very repetitive. If you have enough helpers it can be done at the same time as laying the Bidim (see below), but not earlier. We must do it after weíve cut and assembled the ag-pipe into the manifold. This is so we can use up the leftovers of the rolls and any wrongly cut pieces of ag-pipe, in making these clips.

These simple but clever clips were invented by James Hill who helped install the second swimming pond of this type in Australia (the one owned by Clare Rudkin). Heís the one crouching down installing them in the photograph in the installtion section below. These clips allow us to tie down the "boyproofing" Ė the plastic mesh that prevents active feet from churning up the gravel (the polyethylene mesh would otherwise float and non-floating meshes such as PVC, rubber or expanded aluminium are way too expensive). And these clips tie down the mesh in a way that does not require us to make any holes in the Bidim (which would let gravel through).

The clips are made from a 100 mm length of ag-pipe and a 200 mm cable-tie. Slit the piece of ag-pipe along its length with a stanley knife (taking great care never to have any body parts in the line of fire, in case you slip) and thread the cable-tie in thru one drainage slot and out through the next, in the middle of the side opposite the slit. Thatís all there is to it!

Of course children should not use a stanley knife, but even quite young ones can help by threading the cable ties. It is nice to give them a sense of pride in having contributed, and provided you are patient with them it will leave you with some good memories too.

The clip will be spread apart to fit over both bidim and ag-pipe branch. They are positioned on every second branch and so will be on a 460 mm grid. You will therefore need about 5 of them per square metre of bottom. The area of an oval pool can be calculated as approximately (length Ė 3/14 x width) x width. So with 9.6 m x 4.5 m we have about 38.9 square metres and therefore weíll need about 38.9 * 5 ~= 195 clips. Make a few extra since they sometimes break while being put on. Say 200.

Lay the bidim

Take the previously cut and marked lengths of Bidim and, starting with the middle one, unroll it over the ag-pipes while ensuring that enough is bunched between each pair of ag-pipes so the marks fall on the tops of the ag-pipes. This is much easier with two people, one at each end of the roll. There should also be approximately 300 mm of bidim up the walls. You can hold this in place with occasional pieces of duct tape.

Then take the next length of bidim and do the same thing, overlapping it about 100 mm over the previous length. When the bidim covers a trunk, ensure that enough of a tuck is taken in that direction as well, so as to allow it to reach the bottom on both sides of the trunk.

After laying the five lengths of bidim go back and tuck the bidim around the ag-pipes while ensuring that the ag-pipes are arranged reasonably straight and parallel. Using strong sharp scissors (with rounded ends to avoid accidental liner damage) cut the bidim to fit the oval shape of the pond, leaving about 300 mm up the walls and duct taping it there. Use the offcuts to finish the ends.

Where the bidim overlaps the pump intakes, cut the Bidim in an 8-way star to allow it to fit tightly over the pump intake and use two linked 300 mm cable-ties tied tightly around the intake and Bidim (and if itís dry, wrap duct tape around it a couple of times) to prevent gravel getting under the Bidim. And while youíre at it, you could put some duct tape over the opening to prevent any gravel getting in there when you start putting the gravel in place.

Wetting the Bidim by spraying it with a hose, helps to keep it and the ag-pipes in place until we get to putting gravel on it, and keeps your feet cool.

Install the Hill clips

Come up with a plan for laying the mesh, to minimise the amount of cutting required. You shouldnít take the mesh all the way to the walls (within about 200 mm to 400 mm is fine) and you donít need to take it into the refuge area, although you can. So you can probably get to use all full-width runs and only have to cut them to length. It is good to have the edges (including where different runs meet side-by-side) over ag-pipe so they can be tied down to it. When youíve worked out the mesh laying plan, you will know where you need to install the clips

To install the clips, start in the middle and work your way out, clipping them onto the ag-pipes on an approximately 460 mm square grid. Position some right on the ends of outermost ag-pipes. Note that the photo doesnít show the required density of clips.

Install the side-wall protector

This only applies to ponds with a plastic membrane liner.

Remove the coping from around the pool edge. This is sometimes very difficult when a pool has been in place for some years. Liberal spraying of screws with penetrating oil such as WD-40 is usually essential.

Some pools have separate clips or rails to hold the liner to the tops of the walls. With others itís the coping that does this. You should figure out whether itís necessary and possible to clamp both the liner and the protector (and the UV mini-skirt if this is separate) under these clips or rails or whether the liner only should be under these and you rely on the coping to hold the protector in place. If the latter, then you may need to use duct tape to the outside of the pool to hold the protector up, at least until you get the coping back in place.

Start from the middle of the side where the fish and plant refuge and risers will be, and unroll the previously cut and joined protector inside the pond. Allow it to curve inward slightly at the bottom so it just overlaps the ag-pipes. This is so that the action of the pump will tend to remove water from between this protector and the waterproof liner, so it doesnít bag inwards and look unsightly. Fold whateverís left over the top of the wall.

When youíve gone right around and overlapped the start, you should go back around and flatten everything out as well as possible, both inside and outside. Then trim the protector so the overlap is only about 100 mm and join it (with Selleys "All Clear" for builders plastic (polyethylene) or appropriate adhesive for other materials).

Then do the same with the UV-protective mini-skirt if this is a separate item. You should ensure this extends at least 100 mm below the lowest water level you expect to allow and wraps over the top of the wall by at least 100 mm.

Reinstall the coping around the pool edge.

Stick the bottom of this PVC mini-skirt down to the polyethylene liner-protector with occasional spots of "All Clear" so that it does not get annoyingly lifted up and away from the wall by turbulence when people are swimming.

Add gravel to 10 mm above pipes.

This is the exhausting part (Thanks Brendan). Unless you can figure a way to use machinery without risking damage to the pool, youíre going to want as many helpers as you can get. Some to fill buckets or wheelbarrows, some to carry them to the pool coping, and some to spread them inside the pool. Start by pouring gravel in lines at right angles to the ag-pipes to maintain their even spacing. Donít just dump a big amount in one place causing the pipes to spread in that place and bunch up in others. If using a wheelbarrow and ramp, first fill evenly between the pipes in the area where the wheelbarrow will dump.

At this stage, donít cover the bottom of the side liner-protector with gravel, but carefully put the gravel under it while making sure no gravel goes outside the Bidim and against the liner. The Bidim should have been duct-taped to the wall at intervals.

Continue until you have a roughly level surface all over and cannot see any Bidim (geotextile). The tops of the Hill clips should still be visible and there should be no more than about 10 mm of gravel over the Bidim on the tops of the ag-pipes and trunks.

Wash the first layer of gravel

Even though your gravel should have been described as "washed" and may look reasonably clean, I still recommend these in-pond washes, unless youíre really short of water. And if youíre really short of water, why are you building a swimming pond? Donít bother trying to wash it while itís in a heap outside the pond, it doesnít work. Only a thin layer gets washed.

Wrap a piece of flyscreen or a scrap of Bidim around a basket or milk-crate or orchid pot or some other rigid container with holes, to make an intake filter for pumping or siphoning out the dirty water without sucking up gravel. Dig a hole in the gravel with your hands, somewhere near the middle of the pond, or at a low spot if you remember where one was (e.g. the last puddle that you emptied), and put the makeshift intake filter in this hole. Use a hose (or hoses) with focussing nozzles to blast water down into the gravel. If you saved any of the original water from the pond, and have a suitable pump and fittings, now is the time to use some. While doing this, use the water as a level and put more gravel into any low spots.

Work over the whole area until the gravel is covered with about 50 mm of water. Then start the pump or siphon to drain it. Prime the pump or siphon with a hose (without nozzle) up the intake pipe and temporarily sealed to it with your hand and held under the dirty water. Use a brick or two to hold the intake down and prevent it forming a vortex and sucking air for as long as possible.

Donít be tempted to pump out via the undergravel filter. You will just clog the bidim with dirt. In fact it may be a good idea to run some clean water into the undergravel filter trunks while pumping out.

While emptying, again make use of the water as a level to show you where the high and low spots are and use a plastic rake or broom to level the surface of the gravel.

Lay the mesh and tie it down

You should have worked out a plan for laying the mesh before you installed the Hill clips. Remember, you donít take the mesh all the way to the walls (within about 200 mm to 400 mm is fine) and you donít need to take it into the refuge area.

Make sure the mesh is laid so the curl off the roll makes the ends curl downward. Have someone hold the roll on the rake handle just outside the pond coping so you can pull it out into the pond and cut it to length. Secatuers work well for the initial rough cut. If in doubt, make it a bit longer, you can always trim it down later. Cut all lengths before tying any of them down. Then first tying them to each other side-by side with 100 mm cable ties. Then tie the whole sheet of mesh down with the cable ties of the Hill clips. Donít cut off any cable ties; that tends to leave sharp corners. Just tuck the ends underneath the mesh.

Donít be tempted to use a flame to melt the sharp cut edges of the plastic mesh. This would be good for safety and comfort but it causes shrinkage of the heated parts, which makes the mesh curve. This makes it dome up in some places instead of lying flat and then you need a lot more gravel to cover it, just to cover the high spots.

Just ensure that you always cut close to a crossing strand, using large sidecutters, so there are no ends sticking out. That means that where you might have initially cut smooth curves they will have to be "pixelated".

Add the rest of the gravel.

Start by putting gravel on the bottom ends of the liner-protector, all around the walls, and pulling it tight. Remember that the protector must partly overlap some ag-pipes so that water will be pumped out of the space between the protector and the liner.

The aim is to have about 50 mm of gravel over the ag-pipes when it has all settled (after it has been covered in water and walked over for some time) so as to have a roughly even distribution of flow rates at the surface of the gravel. Donít worry too much about the exact coverage, but if the mesh starts showing through in patches later you should add more gravel (although you may find it hard to round up the helpers again). J

In the case shown in the photos, we goofed and put in far too much gravel before we tied the mesh down, as well as having the doming problem due to melting over the cut edges of the mesh. So we didnít have enough gravel left to cover the mesh and had to order more.

Wash the final layer of gravel and start filling the pond

Repeat the process as for the first wash, but this time you will need to dig the hole for the pump or siphon intake, somewhere where there is no mesh. After pumping out this lot of dirty water, you can begin filling the pond.

Make the mesh fence for the fish and plant refuge

The fence consists of the same green polyethylene mesh (with 16 mm square holes) as used on the bottom, but this time tied with plastic cable ties to self-righting posts made of 40 mm PVC pipe with end-caps.

The mesh should go from the gravel on the bottom, up to somewhere between the highest possible water level and the top of the pool coping. Measure these two heights. The mesh is only 920 wide, so work out what fraction of a width you need to add to get something in between these two measurements. In our case it is 1150 mm to the high water level and 1250 to the top of the coping. Using one and one third widths of mesh gives us 920 + 306 = 1226 mm. Weíll assume the fraction of one third from now on.

Measure the distance to be spanned, from coping to coping, and add 200 mm to allow for wrapping the mesh around the two end posts. If in doubt, make it a little longer. You can always cut it shorter later. Cut one full length of mesh then cut another one to one third of this length. Then cut the shorter one into three strips of one third of the width (i.e. approx 306 mm widths) and tie these strips end to end using 100 mm cable ties to make a full length strip. Then use 100 mm cable ties to join the full-width piece and the one-third-width strip together to make a full length of one-and-a-third widths. The joins should go nearer to the bottom of the pond, so they are not noticeable.

The posts should be spaced about 1.0 to 1.5 metres apart, so work out how many you will need. Make the posts evenly spaced, but choose their number so that none is directly in line with a pump and riser.

The two end posts will be tied to the outside of the pool coping to stretch the fence tight, but the others are free standing. Their length is such that their bottom end-cap is buried in the gravel and their top end cap is just above the pool coping. So add the length of two end-caps to the distance from gravel to top of coping and cut 40 mm pipe to this length. Deburr the cuts.

All posts are made self-righting by filling them partly with gravel and water. Maximum self-righting torque is achieved by filling them to half the water height. So the bottom end-caps must be properly sealed. Coat the bottom end-caps inside with PVC solvent cement and push the pipe all the way in. The top end-caps will not be glued or sealed, but only pressed on, to allow for later detection and repair of any leaks or overfilling.

Subtract half the water depth from the length of the pipe and use a tape measure inside the pipe to check when the pipe is sufficiently full of gravel. Fill with water to the top of the gravel (you may need to shine a torch in to see this) then press the top end-caps on.

Wrap the fence around the end posts and tie the mesh to itself with 100 mm cable ties. Put extra ties near the top, where the most tension will be. Tie the fence to each intermediate post with five 200 mm cable ties, one at top and bottom, one in the middle, then split the gaps in half again. After pusing the end posts down into the gravel in the correct location, use linked 300 mm cable ties or nylon rope or bungy cord to tie the tops of the end posts to some point on the outside of the coping. You may need to drive a screw into the coping to attach them to.

Install the pumps

Donít worry if the water still looks cloudy. It will all come clear when the filters get established. Fill the pond until it is just overflowing (typically out of the skimmer box), as it will sometimes do with rain. Then you can install the pumps and cut the risers to length so that the water level is at the top of their outlet elbows (or UV-head elbows). The water level will drop over time due to evaporation (hopefully not leaks), but if it falls below the bottom of the outlets you should top it up.

Run the double-insulated pump and UV-head cables into a waterproof junction box screwed to a pool post (about halfway between the two pumps in our case). Use silicone to seal the cable entries. Run conduit from this junction box back to the control panel which should be in a sheltered location. 32 mm diameter corrugated conduit is good. You donít have to bury it 600 mm deep as you do 240 volt wiring. In fact you donít have to bury it at all, but itís a good idea to do so.

Use at least 4 mm2 building wire or 6 mm automotive wire (red and black) for wiring each pump (larger if you have to run it more than about 10 metres to the control panel and power supply). At the low voltages and moderate currents used, any significant resistance in the cable will cause a large proportion of the voltage to be dropped along the cable, thereby wasting a lot of power. Cables must always be protected by appropriately sized fuses at the power supply end, to avoid the risk of fire, e.g. if a pump has a short circuit (as they often do at the end of their life).

Each pump should have a separate pair of wires and a separate 6 or 8 amp fuse, but up to 5 UV-heads can be connected in parallel (inside the junction box at the pond) and run back to the control panel on a single circuit like that for each pump.

[More information about wiring up the pumps and UV heads may eventually be included here, or in the documentation for these items and their power supply and control panel.]

Fish Caves

I understand that Firetail Gudgeons, and some other fish, only lay their eggs on the roofs of caves or the undersides of leaves, and they also need caves to hide in during the day so they donít get too stressed. Old terracotta sewer pipes are good if you can get them. I also recall seeing hexagonal terracotta pipes that could be stacked to make wine racks, but I couldnít find any of these either.

I ended up using terracotta orchid pots (thanks Clare). These make good fish caves when placed upside down on the bottom because they have several large holes around the sides which fish can swim through. Remove any sharp burrs from the holes with a round or half-round file. Wash them and remove any labels before placing them upside down at regular intervals along the bottom of the fish and plant refuge.

Do not add fish until the system is completed, including plants, and has been running for at least two weeks. There will be wild oscillations in water quality for some time as various algal and bacterial civilisations rise and fall, but eventually some kind of equilibrium will be established. The rule with managing ecologies seems to be, "Donít make any sudden moves". But what could be more sudden than creating one, all at once, where it didnít exist before.


This is unfortunately the least well developed part of the design. Algae, Bacteria, other microscopic forms of life, and on up to insects, will all find their own way to your pond. Word soon gets around. But if you want to have any chance that more readily harvestable flowering plants will take up the nutrients from the water and reduce the growth of less readily harvestable filamentous algae or cyanobacteria (blue-green "algae"), then you have to add them.

Most water plants do not like to grow at the depth of a typical swimming pool. Iíve mostly used Ceratophyllum demersum (foxtail) because it is a free-floating submerged plant, and Iím currently trialling two deep-planted Australian Vallisneria species at the suggestion of Nick Romanowski.

He has also suggested the emergent Phylydrum and others, but for those we need to build some kind of raised shelf or platform inside the refuge; one that gives us some gravel to plant into at only about 200 mm below the water surface. In the case of an in-ground pool, this could be a dug-out and plastic-lined area beside the pool.

I suspect that much of the reason the plants are not currently very effective in preventing the growth of algae is that there is nothing to constrain the water leaving the filter to go past the plants before returning to the swimming area. A raised shelf or trough could also serve this purpose by having a raised edge above water level that only allows the water to return to the swimming area after it has passed the plants. Any ideas on how to construct such a thing cheaply would be greatly appreciated.

Ultimately I suspect the best solution will be to use algae itself, as in the algal turf scrubber invented in 1991 by Dr Walter H. Adey in the USA. See

The end.