Aquaclear filters are awesome! I run a 50 on my 29g along with a canister.
Have to agree the 30 or 50 would be fine for a 20high, they do have adjustable flow and there are methods to baffle the outflow to reduce current if you need to.
I had a AC 50 on a 20 long community tank with 2 female bettas. For the bettas I had a patch of water lettuce that I kept trimmed to about 6x6 inches. They layed in it, and would venture out when they wanted. They loved it, and had no issues.