The Koi Pond Project 2019

A brand new Pond and Filter System built from scratch.


Full details below, but if you want to see a videos of the Pond now it's mature, click here: Pond Video

And for the Filter System, click here: Filter System Video



Even the final version (Mk 3) of the Koi pond at the last house had its limitations.

Daily maintenance requirements of the filter screens (to prevent potential blockage) and bottom drain flushing and the potential disaster of power cuts (not that uncommon) meant that being away from home was never as totally relaxing as it should have been.


Moving to a new home without a pond for the Koi meant selling them with the previous house and now being "koiless" a reappraisal of the whole fish keeping process followed and for several years it was decided to do without. Apart from anything else, now we were on a water meter the very high water turnover of my previous system would have made the hobby even more expensive.


In 2018 we decided to investigate the possibilities of a new Koi pond taking advantage of advances in filter technology. If we were going to face the considerable expense and major work to build a complete new pond and filter system I wanted to get it right!

I developed a set of principles to operate to:

The system had to be capable of:

       * Running unattended for up to 21 days.

       * Coping with RCD circuit failure or total power failure for at least 4 days.

       * Coping with main circulation pump failure for at least 4 days (or indefinitely if power available).

       * Only changing/topping up water as absolutely necessary.

       * Maintaining a constant water level without intervention.

       * Remaining frost free but only heating to protect against extreme cold.

       * Keeping the aesthetics of the pond like an ornamental pond and not an outdoor fish tank.

       * Keeping the water chemistry ideal for Koi and the water clarity ideal for human viewers of Koi!



The Pond Construction

The pond design I chose would be formal - again.

I had considered a more natural look, but the problem with a natural look would be acquiring and moving by hand suitable sized rocks (big rocks are always better!) and making a waterproof fish friendly smooth edge. I did consider trying to produce artificial rock in situ (geology degree put to use at last?), but this would be yet more time and complexity added to what was already growing into another major undertaking.

One thing the new pond was not going to be was a "traditional modern koi pond", which to me is a rectangular semi raised external fish tank, often with a window. Don't get me wrong, many of these are beautifully designed and built, but jointly it was decided that I would build an ornamental garden pond that just happened to be a "stealth" koi pond. So no visible pipes, skimmers, air lines or columns of bubbles.

This pond would have vertical walls with overhanging slabs, have a bottom which at every point was sloping towards a bottom drain, be rendered and fibreglassed - like the last one.

The depth was partly limited by a layer of ironstone I hit, which meant I could only put up with so much pick axing.

The only excavator I would be able to get in to the pond area would be useless on the uneven ground - I have used very small ones before. More often than not in difficult ground the bucket stays still and the digger moves. This pond was to be dug out by hand - every last barrow full. I did the whole thing solo apart from the fibreglassing this time.


I had just finished the scratch built log cabin and the starting point (now Spring 2019) was pretty daunting!





In the photo below (left side) you can see the reinforced concrete retaining wall necessary to give the levels wanted in this sloping garden.





Below you can see the excavation (all by hand remember) down to ring beam level.

The ring beam is a reinforced concrete foundation around the pond onto which the vertical walls are built.





With the ring beam in place the block walls are built. These are to achieve straight vertical walls and to act as shuttering for yet more reinforced concrete (rather than be structural in themselves). Numerous stainless ties will lock the walls into the reinforced concrete.





Once the walls were built it was a case of digging out and shaping the base of the pond which ended up at about 1.5m at the deepest point (internal depth allowing for base). To some this may appear very shallow, my last pond was 1.8m+, but extensive research suggested all would be fine and I had had enough of pick axing!. The floor is covered in about 70mm of sharp sand render and finally normal render. Minor cracks won't be an issue as the whole thing will be fibreglass lined. I used this construction method because it is how I built previous ponds and is the same basic principle as used in liner swimming pools. I know this because I built such a pool in a "previous life".


Below you can see the finished structure - note the thickness of the reinforced concrete - this pond is here to stay! The thickness of the concrete is partly to act as a solid base for the edging slabs.

You can also see the skimmer, main return (direction adjustable - a nozzle made from a 110mm bend directs water down in the Summer and up in the Winter to disturb the surface and discourage freezing) and the "reverse bottom drain" (as mentioned elsewhere, this design won't get blocked - if it can suck it up it can suck it through).





More photos with the temporary roof required by the fibreglass company. I did my own fibreglass work on my previous pond but time was of the essence here - the fish had been imported, bought as a job lot and we needed to get the pond up and running and established well before the Winter.








The Filter Room


The filter "house" would be in the dip below the new en-suite window. This area also benefitted from a water supply "T'd" from the new supply to the log cabin. The strange snaking wall was already there (circa 1971).





Below you can see some of the pipework to sit beneath the filter house base.

Note the DIY bending creases - cheaper than joints and will never leak!

Controversial! You will see many times on the internet dire warnings about using ring seal 110mm pipework where you can't get access after construction. I have always used this type of fitting without a problem, but still mindful of the ongoing warnings I spoke to the manufacturer to double check. They said that the ring seals would easily handle the water pressures involved (and much more), would allow some movement without splitting or breaking the seal (which can happen with solvent weld) and if they did ever leak it would be a tiny seep - into well drained soil.........





The whole system would be in an insulated custom built "shed" (note the rather grand £15 second hand UPVC door).

The shed would be on a reinforced concrete base (below ground level with a degree of tanking) and have concrete block dwarf walls (higher at the back to accommodate built in chambers).

You can also see more of my retaining walls and home made fence panels. I went to some lengths to manipulate ground levels to create a private space around the pond area.






The Pond Filter System

Below are details of the filter system.

I have also uploaded a YouTube video which can be found here: Filter Video


This latest version of the water treatment plant features a number of processes and systems in which I have tried to include current thinking in effectively dealing with fish excretion and waste while reducing energy and water consumption.

Energy savings can be measure immediately, but the biological processes take much longer to evaluate.

Below is an overall view of the Filter Room.




       1. Inlet chamber:

Gravity fed Skimmer and Bottom drain inlets. Being used to using homemade stand pipe "valves" and being happy with their simplicity, reliability, versatility and low cost I decided to use these again rather than taking the ball or gate valve route.

110mm Slotted stand pipes in from Bottom Drain and Skimmer. Can be adjusted or shut off.

This is the ideal place to take pond water samples for routine testing.


       2. Profidrum filter and fluid media chamber with UVC.

Twin 110 mm pipes to Profidrum Bio self-cleaning drum filter with UVC and aerated fluid media bed (supplied by Absolute Koi). I looked at all the drum filters on offer and this one seemed the best engineered of the bunch - the whole idea is that these look after themselves and I wanted to have complete faith in this. Regular maintenance is reduced to checking the waste chute and running and checking the rinse cycle manually from time to time.

The Profidrum has been modified as follows:

1. Baffles to prevent UVC light shining into the moving bed chamber.

2. An automatic 40mm float controlled drum bypass in case of blockage for any reason.

3. Sliding plate and new ports to allow variable bypass of the moving bed to feed the Anoxic part of the system raw pond water.

Most water bypassing Moving Bed.

Most water going through Moving Bed.


       3. Air Lift Pump.

Twin 110mm pipes again lead to a 250mm diameter sump pipe into which is dropped an Air Lift.




The Air Lift has two major advantages for this application. First, 55W can produce the flow rate equivalent to approximately 160-170W impeller pump (my own comparison measurements). Second, the Air Lift aerates the water more than any other system.

The downsides are limited vertical lift (in this system less than 20cm at all times) which means designing around this and having to use gravity returns. Neither are downsides if designed in, but can be if considering a retrofit.



4. Anoxic Chamber.

I spent a lot of time working out how best to do this in the space available.

Eventually I had a plain Polypropylene tank made (12mm sheet thickness).

This I braced with stainless steel rods, which also act as supports for the Biocenesis Baskets.



This chamber holds 15 baskets (5 rows, three vertically) and my repurposed BHM.







       5. Outlet chamber :

The main flow all ends up in the outlet chamber. In this chamber also sits the pump for the water feature (P3), placed here because whether on/off the flow through, and therefore the levels within the filter remain unchanged (all that changes is the clean water route back to the pond).

Also in the outlet chamber is the ability (screw on 110mm caps) to send water back to the pond or back round the filter (now isolated from the pond) to allow pond treatments (such as KMnO4) without affecting the filter biofilms.

There is also a weir to overflow back into the inlet chamber should there be any blockage in the return.

The small black pipe is the outlet from the auto top up.

In the Winter I also drop my floating water heaters into this chamber (the 4 x 600W heaters sit in a large float pad to hold them in place).


Auto Top Up:

Totally separate is a level control with its own dedicated 40mm pipe connection to the pond and float/overflow chamber. This will accurately control water level and be totally unaffected by the often varied level in the filter. It is filled via a digital water meter and discharges into the outlet chamber through a dechlorinator. It also features an adjustable overflow weir. It does not add water by the float valve as the long narrow bore connection to the pond would cause interference with accurate float level.


9. Temperature control

None of my previous ponds have been heated. For this latest pond I researched this topic at length and as is so often found myself gravitating back to the wisdom of Manky Sanke. The website to which I imagine all enthusiasts go for water quality enlightenment.

The heating regime would need to be decided before pond construction because the type of regime would determine whether to insulate or not.

As has been mentioned, this particular build had a number of "aesthetic constraints" placed upon it and one of these was no covering in Winter.

This alone guided me to the "basic protection" regime of keeping the water temperature above 4C over the Winter. An uncovered pond at a much higher temperature than ambient would cost a fortune to heat!

With this in mind, pond insulation would be counterproductive as the subsoil temperature is well above 4C all year round and I want some of that "heat" to be conducted into the water as the temperature gets very low.

The next choice was how to heat the water when required.

For the relatively low temperatures I want to maintain there was no point considering a large complex arrangement with boilers, heat pumps or similar (been there done that with swimming pool heating).

I retained an old fashioned element to the filter system, that is inlet and outlet chambers (of which I am a fan for many reasons).

This allowed me to buy inexpensive floating heaters and group them in the outlet chamber. Floating heaters usually have a problem in that they have a relatively low temperature cut out. Fine if all you want to do is keep a small area ice free, but less fine if you want to heat a larger volume of water. Fortunately the rapid water turnover flowing around the heater elements in the outlet chamber overcomes this problem. This meant I was able to by 4 X 600W heaters, giving up to 2400W for £80.

Temperature is measured using a standard STC-1000 unit, calibrated accordingly.

WHat happens when the water temperature drops below 5.5C in my system?

1. Water heaters turn on (2.4kW)

2. Soil heating cable wrapped around mains water supply pipes turns on.

3. My pump house is less than a metre from the house, so a small fan draws warm air from the house into the filter shed through the same 110mm pipe which carries the power, etc.


In the picture below you can see the fan in the centre (hidden behind anti-backdraught flap) and the green heating wire on the water supply.

Please note - this shows an earlier version of filter system, but the heating parts are unchanged.



And here are the heaters in the outlet chamber:





Filter schematic


Please note that the back up air pump is now a back up impeller pump. This is because drum issues could cause a level drop in the sump and mean an air lift might not manage the head.




Monitoring and Back Up Systems


This is a key part of the design. If a part fails and I am not about I donít want to end up with a disaster. At my previous house I came back from a week away to find several of my larger fish dead. The pump had failed the filter had stagnated and I don't want to go through that again.



RCD trips on ring main

Power cut

Main Air Pump fail

Drum fail/drum spray fail

Backups fail




Relay automatically switches to other RCD circuit

Relay engages 220Ah battery backup to backup pump with inverter.

Relay engages 220Ah battery backup to backup pump with inverter and operates charger to maintain.

Water level in pump chamber would drop and so main pump would switch off. Backup pump kicks in with much lower flow rate which would operate even if drum completely blocked because I built in a float controlled bypass.

CCTV monitors key pump displays. CCTV is remotely accessible. As a final safety measure I installed a small module that detects when the main pump has stopped (for whatever reason) and sends a "Pump off" text to my mobile (and helpfully a "Pump on" text if it starts up again after a power cut). This has its own low cost sim card/contract. I then activate my backup backup (I phone my son who lives locally).



The following photo shows the 12V backup system (pump is a 25W 240V AC unit, Inverter fed, within Air Lift sump pipe).

The high level exterior vent (soil pipe bend) is in case of Hydrogen gas build up from charging the batteries (which are meant to be sealed, but better safe than sorry).

And this shows the main system displays, from left to right:

Water feature pump, water temperature monitoring and control, back up pump (only displays when running) and main Air Lift air pump display.

Also visible to the left of the temperature display are the two separate power supply circuits and changeover relay.

The Finished Pond

The photos are early 2020 during lockdown V1.0. Will take more pictures in Spring 2021 when it's all matured a bit.



As you can see there is little evidence that this is a fully filtered Koi pond. Most visitors assume the only "filtration" is the water feature. Even the skimmer is hidden under a hinged slab.

The water feature main parts are made from a sheet of high quality stainless steel I got a local engineer to bend on their machine and an old round granite table top I cut up.







And moving forward to 2022 and you can see how the area has matured.