I switched to an air-driven sponge filter for my cherry shrimp tank (from a Fluval underwater sponge filter) and water conditions have improved. I used to have a lot of surface scum and now none, probably due to the bubbles breaking the surface.
Also this is easier to clean since I had to put pantyhose around the fluval to keep the shrimp out of the impeller.
My only complaint is the noise, both from the air pump (Whisper 20) and the bubbling. Plop, plop, plop all the time. If I could make the bubbles smaller they wouldn't make noise. Any suggestions how to do that?
BTW, my other air pump is a "Million Air MA600" and is quieter, so if anyone is shopping I'd recommend that one.