I have a home-made wet dry made with a rubbermaid container ( 38 gal) that I have placed a bulkhead in and cut out the lid to fit over an old commercial food container full of bio balls. I am running the box of balls about2 inches from the eater line but i do keep my sump standing with about 6 inches of water in it for reasons of top off. I havent heard of the balls being submersed in the water but it may be being done. My water had always tested crystal clear and without problems