The only fish in my 20 gallon high currently are one 3" BN pleco, one little Ram and one shrimp. That tank gets dirtier than the 55 gallon cichlid tank by far - I have to vacuum the gravel at least twice weekly.
I also have a tank with a bunch of frogs, but they are the cleanest underwater critters I've ever had. Have never seen any of them poop, and never see it in the tank. It's weird, in a good way.