They are a catfish, some LFS have them, some dont. They look like a twig. They aren't even 1/2" wide, and maybe 3 inches long. They dont produce a lot of waste. I had mine in a 10 gallon for months before I upgraded to a 40 gallon. It seemed fine. I think a 20 gallon would be OK. But I'm sure there are experts here that would know for sure