Do you not have the room for something bigger?
Honestly he'd be better off in a very large tub with filtration than a 20g tank. That's not much.
A large tub with some filters could be set up very cheap and give him room to move around, and an easier time maintaining water quality.
As far as food, he'll eat a variety of things. Slices of zucchini, algae wafers, just about any type of pellet, etc.