I keep 2 BN, one is 6" (in a 20L) and the other 4"(in 20H), but also need driftwood and blanched veggies for them to munch on. I wouldn't do a BN in a 20g unless you can keep up with large weekly changes or know how to keep big fish healthy and happy. That takes a few years to understood, when we start off, we just put any fish in a small tank without knowing their requirements until later on. It's live and learn in the hobby, I've done that in the past, but now I keep different fish in 3 tanks without any issues and I do research before I get the fish I want.