Not sure why everyone on here is always negative, let the guy try what he wants. I have two 3 inch redbreast sunfish with a 3 inch green terror, pictus cat and silver tip shark in a 50 gallon , it's of course a grow out tank as is yours. (who is a more aggressive than any other gt I've had) so I would agree with a convict. And of course, when the time comes, upgrade to 75 gallon or larger. I think a 20 gallon is fine for now. I had the toe sunfish and the get all in a 10 gallon for almost 2 months with no problems