What pet can I get that will be comfortable in a 20-gallon tank?

I found a 10 gallon tank and a 20 gallon tank lying around in my basement, and now I'm inspired to get a cool pet. I think both of the tanks are too small for a turtle or a tortoise. Could I get an aquatic frog or a regular frog? Might get some fish to fill the 10 gallon tank. In your opinion, what pets can I get that will be comfortable in a 10 or 20 gallon tank?