Bottom drain for koi pond:
Koi ponds are designed for koi keeping and as such, high water quality is the number-one goal. Large external filters are the norm and drains in bare bottoms are considered a necessity to deal with large fish loads and optimum water circulation.
Koi ponds are usually smooth, bare liner or concrete bottoms, so debris can be easily removed as it slides down along the slope toward the drain. Drain sizes vary from 2 inch to 4 inch in diameter depending on the pumps used and pond volume.
The AKCA Guide to Pond Construction recommends a minimum slope of 1 inch drop per one linear foot, but a 2 inch drop/foot is preferred. This guide notes that a drain with the proper slope will draw water from a 4' to 6' radius. Multiple drains should therefore be used based on this area guide so all parts of the pond are drained adequately.