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I hope I can get some help on my question but at the very least maybe someone can direct me to a site that can.

This question relates to size of koi pool and fitering system used. We are looking to build a koi pool of about 20,000 litres (about 4400 UK gallons) and I'm trying to wade (no pun intended) through all the different filter systems that are available. I'm not going to ask what the best filter system is (although I'm open to suggestion) I'm just asking how I calculate the filter system size and waste water throughput for a given number of mature koi (say) 40.

I'm also considering running two smaller filters in parallel i.e. operating independently. What are the pros and cons of this?

I hope this is the place to ask these questions and if not please do redirect me elsewhere

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I cannot answer all of your question but have these observations from my own experiences keeping koi:

  • the bigger the pond the better. Sheer quantity of water provides a buffer for temperature changes and water quality changes
  • koi are very happy to eat anything they can get in their mouths. That prized water lily, that rare native water plant: a mere snack while they think about eating something else. You will be fortunate if you can keep any plants alive in the pond area. Without the help of plants you will need to rely completely on artificial filtration.
  • for filtration, more is better, you cannot have too much.
  • A good basic filter design will incorporate at least 3 chambers - settlement, polishing and biological
  • an empty vortex chamber which allows debris to settle is an important part of a good design. Just having a filter pad a the start of the process will quickly clog up and require cleaning as well as slowing down your flow rate.
  • it is important that the water that goes through your filter system spends enough time in the filtration. A quick how-do-you-do, out-you-go doesn't let the system work
  • the biggest challenges for maintaining water quality are in keeping the same quality over time. As a pond ages organic debris builds up, algae will start and any change in seasons adds to your challenges. The industry standard for algae control is a UV filter. Many products with differing capacities are available.
  • one store I know that supplies and installs ponds for koi insist that the only way to go in the design phase is to have a drain at the bottom of the pond which allows for easy removal of debris. I did not install one in my 5000 liter pond so I end up doing mung duty twice a year to vacuum out the mung.
  • another part of design is planning for failure. I live where having a heater in the pond in the winter is essential. If you only have one heater and nothing ever goes wrong you will be fine. You should have a backup pump, heater and spares of all critical infrastructure just in case of failure.
  • I found this site to be most informative. Note the placement of outlets and jets to self clean the sides of the pond. I also note the diagrams for ponds of the size you are talking about seem to resemble plans for a municipal filtration plant. Good luck, its a complicated project worthy of attention to detail.

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