There are a vast number of cats that won't exceed the 6" in length criteria you are demanding for a 20 gal tank. As for the 110 gal pond... that is a great holding tub for all the 6" cats you want to stick in the 20 gal.
Cats are happier when they got the room to explore. Hence, the reason why I have a custom tank being built by ATM for my new home. They are just happier in a bigger world!