It is generally accepted that 4 inch bottom drains are a must and should be fitted to all ponds under construction, those being modified, or alterations such as fitting a new fish safe, 45 mil EPDM Pond Gard liner.
The number of bottom drains required will depend on the size and shape of the pond, and also the preferences of the owner.
Some 4" bottom drains even come with aeration. Aerated bottom drains must have an air pump to work. As the drain gives some oxygen back to the water column it also gives off tremendous circulation and sweeping action of debris back to the bottom drain opening and on to the biological filter.
Bottom drains are designed to be placed at the deepest part of the pond for the removal of the waste given off by the koi as well as air born debris like leaves and twigs.
Bottom drains count for the vast majority of the removal of waste products in a Koi pond when compared to that of a surface skimmer.
The vast majority of solids will sink to the bottom of the pond in time, where ideally, they will be sucked into the bottom drain and fed to the filter systems where they will be either extracted from the water column by the vortex chamber or converted to nitrate by the nitrification process.
As a general rule, one bottom drain for 5,000 to 6,000 gallons. Of course, the size of your pond will have a lot to do with this - as will the flow rate you want through your filter systems.
Bottom drains - or the pipe work from the drain to the settlement chamber should not be too long. This is because although there is water flow through the pipe work, the heavier solids tend to settle out and can build up in the pipe work itself.
