One of the main reasons for a bottom drain is to capture and remove the sinking organic material before it decays and creates a mulm at the bottom that will end being a great home to pathogens. You want a bottom drain in the bottom of a pond for the same reason as your toilet has a hole. Both capture and remove organic wastes (flushing) before they decay and create nasty health problems. So the bottom line is that a bottom drain will be one of the best ways to keep your pond healthy, thus healthy Koi.
Below are some guidelines for gravity flow rates on bottom drain pipe sizing:
- 3" pipe should optimally have a max flow rate of approximately 1800 gallons per hour and will cover an area around drain 8' radius.
- 4" pipe should optimally have a max flow rate of approximately 3400 gallons per hour and will cover an area around drain 10' radius.
If your pond for example is 9'x12' one bottom drain would work with a 4" line. Now with 3" lines then a total of 2 drains would be ideal.
Now if you have a pond for example of 12"x 20' then we recommend 2 drains if using 4" pipe. Now with 3" lines then a total of 3 drains would be ideal.
Don't forget the TPR's and GPR's in aiding the movement of debris to drains.