The Koi Pond Project 2019

A brand new Pond and Filter System built from scratch.


Even the final version of the Koi pond in Arthingworth 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 a reappraisal of the whole fish keeping process 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 5 days.

       * Coping with main pump failure for at least 5 days.

       * Only changing/topping up water as absolutely necessary.

       * Maintaining a constant water level without intervention.

       * Remaining frost free.

       * Keeping the aesthetics of the pond like a pond and not a 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.

This pond would have vertical walls with overhanging slabs, 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.


I had just finished the 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. 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 the much bigger swimming pool at the previous house.

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) 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 they 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 System


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).






       1. Inlet chamber (on right - outlet chamber on left):

Gravity fed Skimmer and Bottom drain inlets. Being used to using stand pipes 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.

       2. Profidrum filter and fluid media chamber with UVC.

Twin 110 mm pipes to Profidrum Bio self-cleaning drum filter with UV 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.


       3. Pump chamber.

Twin 110mm pipes to chamber housing submersible main system pump (P1) and submersible backup pump (P2). Again, rather than the complexity of dry pumps and associated valves for servicing I went with simplicity. To service, just undo the pipe and lift out, no valves, no bypass, easy.



4. Bakki House Media chambers.

50mm flexible hose to 50mm rigid pipe (incorporating a flow switch) leads to a 4 chamber system (2x2 in parallel) 2x sprayed for maximum aeration and 2x submersed for maximum coverage and to encourage more denitrifying bacteria. Each chamber is an agricultural feeding trough (very strong) with a full box of BHM in each. (BHM Bakki House Media - reportedly the best - and of course most expensive media of its type)

5.       5. Foam Fractionator (Tornado skimmer).

This would run of an adjustable T off the main flow.

       6. Ozone generator into the main flow.


       7. Outlet chamber.

The main flow (with added DIY venturi) and foam fractionator flow all end 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 to allow pond treatments (such as KMnO4) without affecting the filter.

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


8. Level control.

Totally separate is a level control with its own 40mm pipe 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 does not discharge by the float valve as the long narrow bore connection to the pond would cause interference with accurate float level.


Filter schematic





Redundancy and backup.

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.



RCD trips on ring main

Power cut

Main 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 unless drum completely blocked.

CCTV monitors key pump displays. CCTV is remotely accessible so could identify and call out the backup backup (my son).


The following photo shows the 12V backup system (pump is low wattage 240V AC powered by an inverter).


The Finished Pond

The photos are early 2020. Will take more picture when plants and fish more active.