I have 2 fancy fantails in a 20 gal tank. Too small, I know, but it's well over-filtered (2-aqueon 20's). They are pretty small right now-2+ inchs each, and will find themselves in a bigger home when I can find the time and room. They are actually less messy than I thought they would be. Their water stays very clean and the filters cope without a problem. Even if I don't change the water for 2 weeks the tank stays pretty clean. I also make sure not to over-feed them and like I said, they like peas alot.