Bottom drains count for the vast majority of the water circulation in a pond when compared to that of a surface skimmer.
The majority of solids will sink down to the bottom of the pond in time. The solids will be sucked into the bottom drain and fed to the filter system where it will be extracted from the pond system.
As a general rule, one bottom drain for the first 5,000 gallons and then one drain for every 3,000 to 5,000 gallons thereafter. 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 system.
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.
Aerated Bottom Drains give massive amounts of air to the pond and improves removal of solid waste through the drain. Aerated Bottom Drains replace "standard" bottom drains and are the ultimate solution to bottom filtration.