I know it's been said before but fire belly toads would my first option they use land and water are great eaters and are super cool looking mine were amazing just wash your hands after holding them there really toxic my eyes would burn like hell if i did not wash up after handling them. I guess fire belly newts would work but when i had them they were fully aquatic except for perhaps once i saw them out of the water. Among others leopard frogs and bull frogs might work a 20 gal is way to small for a bull frog works for leopards though at least for mine it did. I don't think pacman frogs would work well now i have never kept them but they seem to loaf in the dirt all day the ones i said all are at least kinda active if only sitting in plain sight. Hope i helped and again fire belly toads are your best bet.