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:
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.