The Koi Pond Project(s)

Pond Mark Two "Koi Pond and Filter Mk1"

This was in 1993 so please make allowances for this.

The fish were moved (sounds so easy now!) to a temporary small holding pond next to the main one.

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I took down the walls where there had been movement and completely in the deepest corner. The walls were a hardcore filled and long tied cavity, wide enough to support 600mm square slabs for the pond surround. Unfortunately I have no pictures of this bit, but take my word for it, the pond wall was 6 feet high from foundation to top. Most of it is below "ground level", which is the old pond n the above picture.

The walls were rebuilt with regular wall ties to lock into reinforcing mesh. In this picture you can't really appreciate that from the top of the wall to the outside ground level in the foreground/right is about 5 feet - not small child friendly I accept.

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You can see the huge slope hidden in the shot below. The concrete ring beam goes from 12" thick in the foreground to 72" thick next to the neighbour's wall (hidden in the undergrowth). The soil to the left was built up after the original pond wall was built.

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The cavity was then filled with 20 tons of concrete (mixed and placed in one day by three of us). This wall was not going to fail again.

The next job was to dig down. How far down I hadn't yet decided.

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On a sad note, the above picture shows Alan, my stepfather. He loved nothing better than rolling his sleeves up and getting stuck in. Alan died of an asbestos related illness in 2002.


Seven feet down I hit an old land drain. It was still running and explained why it had taken such a dry Summer for the maple tree to do any damage, despite being so close. I had to make sure the drain was kept open, cover the area with shingle and then a reinforced raft to support the plinth (more later). In the picture below you can also see where the three 4" and one 1.5" pipes are going to leave the pool area.

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This isn't a good picture, but it captures me at "my desk" (undercover photo by my wife through an upstairs window). This is how I do all my designing - in my head. The picture also shows the garden as it used to be, the slopes involved, where I was dumping the spoil and the pre-upstairs extension to the house.

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I was going to build a hopper pool. It would act as a giant vortex settlement chamber with a nifty "suction" drain at the bottom - along with several other inlets/outlets. The design would involve a plinth rising out of the water to hide a skimmer, to house the main outlet, the fountain and the aforementioned bottom drain. Here you can see the pond before the glass fibre. I left it for a week or two and washed the surface with very dilute acid to remove any remaining free "lime". At the top of the picture is the 4" return to the pond (aeration would take place in the filter). This return would feature a directional adapter to get the vortex movement - down for Summer and up for Winter (keeps the ice from forming).

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As I glass fibred the pool I decided to put the seal near the bottom of the plinth. This approach has worked perfectly - that is, the pond doesn't leak. The plank across the top was to hold the anti-shower cover, which saw frequent service! I should point out that the styrene (used to thin the resin while laying) would fill the air in the pond and I had to keep having breaks to "come down"! There are a total of five layers of matting for maximum strength.

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The next stage was to seal the glass fibre. I used a special resin called neopentyl glycol (NPG), which would act as gel coat, colour and chemical barrier all in one. Looks simple enough below, but it was a hot messy job and on the hot day chosen the mixes would tend to over accelerate and catch fire in the pots before we could paint it on. I need to thank Andrew Smith (with me in the picture) for helping out regularly when two pairs of hands were needed.

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I then built up the remainder of the hollow plinth. The back of the structure holds an adjustable skimmer which keeps the water surface beautifully clear.

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The front of the structure has the fountain at the top, the main filter inlet and the bottom drain (at the bottom!).

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One minor problem I have is that I went to great lengths to hide all pipework and inlets/outlets. Consequently when we have our village fund raising "open garden" days I am frequently advised by visitors that we really ought to have some form of filtration in a pond containing koi carp.......

The bottom drain design occupied my mind for some time. If it went down through the base of the pool I would hit the land drain structure, it might get blocked, any number of things could go wrong. Here is my solution.

The "suction" produced by lifting the standpipe in the filter pulls in debris that naturally collects at the bottom of the hopper shaped pond. It is surprisingly powerful. Stones and snail shells present no problem. The gap around the bottom is 25mm, so the fish are also safe. This has now worked perfectly, without fail or blockage for seven years now.

In the picture below you may be able to see the two black PIR sensors for the Heron Scarer....

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Oh, and here are some of the fish. They have all grown since this was taken.

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The filter. 

Have you seen the price of decent sized filters?! I would have to design and build my own or take out a new mortgage.

Key features I wanted to incorporate:

 4" Bottom suction bottom drain, via standpipe chamber to waste.

 4" Lower inlet from pond to filter with adjustable flow regulator.

  4" Skimmer hidden in back of fountain plinth. Height adjustable. 

 Also acts as the higher inlet from the pond to filter in Winter mode.

 Also with flow regulator.

 The fountain pump would be in the filter for ease of access

 (and a filtered  water supply!). 1.5" supply pipe to fountain.

 All sections of filter slope to individual standpipe controlled bottom drains.

 Design allows for potential clogging. 

If away on holiday, filtration would be impaired, but the pump would not run dry.

 4" bypass to allow filter to recycle independently from the pond.

Useful if you need to lower pond level for maintenance.

Useful if you clean the filter and want it to settle before pumping cloudy water into the pond.

Auto water level control (top-up and overflow)


This first picture shows the drains below the filter. The external ones are from the pond and to the sewer (via an air trap!).

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The structure is now building up. The large square area is for fine filter foam. Water goes down through this over the full area. If it tends to clog for any reason, the water simply overflows to the next stage. Flow and pump are thus protected.


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And here is the finished article. Again I've sheathed it in NPG covered glass fibre.

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Since this picture was taken I have changed some of the filter medium. The whole thing is also now undercover.

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The whole project had to be finished by Autumn 1994 so that I could get the fish into their new home before Winter.


Summer 2001 - The Heron Scarer 

We have always seen herons around the village, then during one week I caught one twice by the pond, one by our neighbour's pond and heard that several ponds had been totally cleared of fish. Action was called for.

I had seen scarers that used water and a PIR detector, but these were often fragile, battery powered, ugly and expensive.

My solution -

Two discrete black PIR detectors giving good coverage of the whole pond - but NOT the adjoining patio (the whole thing has a mains power supply).

Frost protected solenoid valve to mains water supply.
Directional pulse sprinkler.
Override switch and auto-arming tied in to house alarm.
Visitors don't even notice it - unless it operates (our cat has learned to take the long way round)

The picture below shows one of the PIR units in a border facing the pond.

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Does it work?

One morning I was awake early, snoozing in bed. Suddenly I heard the "phtt-phtt-phtt" of the sprinkler going off and leapt up to the window to see an enormous Heron rising hurriedly from the now wet pond surround. It works.

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