If you have rocks and gravel in your pond to break down debris, then a bottom drain is not necessary. The beneficial bacteria that seeds within the gravel will remove any build-up that occurs over time.
There’s little difference in oxygen levels two feet down and two inches down, which eliminates one of the arguments favoring bottom drains.
They also tend to promote leaks and raise the possibility of land-locking your koi.
In clean, clear pond water, bottom drains look unnatural and detract aesthetically.
So generally speaking, we endorse the combination of a skimmer, biofalls or wetland filter. In general we discourage the use of bottom drains.
