You really won't need much in the way of biofiltration in a 20g coral prop tank. 10-15lbs of LR
should do it. I'm guessing you'll have a sandbed as well? That will help too. A small HOB
filter would do the job for the occasional dose of carbon and cost you a lot less than a canister. A good fish for a coral growout tank is a yellow tail damsel. They won't bother anything, they're hardy, and corals seem to do better with a fish in the tank. I'd only put one in though. It has worked for me anyway.