What is a swimming fish? Dont all fish swim? If you are talking about feeding him fish, you don't have to do that either... they can eat pellets...
also, why would another fish eat him? are you talking about really big other fish? if so, you don't have them in the 20 Gallon with him do you?
But if you are looking for tank mates, the general rule is nothing with a long flashy tail or is brightly colored. You may have more luck if you get a fast moving fish that doesn't nip. Then they can get away.
and just because your betta doesn't bite you, doesn't necessarily mean they won't go after another fish...
Don't know if that helped at all, since i guess i don't really understand what you were looking for, or the circumstances... or almost anything...