I recently put one of these filters in my 20g low tech planted community tank. Does anyone know what size tank the dual filter is rated for, I can't find this info anywhere? Or does the rating size depend on the air pump rating? I am using a Whisper rated up to 40g. I went for the dual figuring I'd be on the safe side of clean for my tank. This is my first "sponge filter" and so far I really like it!!!