This last one is a tiny guy and only needs a 10g tank, so he'll be happy in your 20... he's easy to take care of as well, but he is a little pricey at $50
Well we just picked up a 1.5yr established 20 gallon tall reef tank. We have some anthelias, and zoas along with 1 clown, 5 snails, and 5 hermits. We are deciding what fish to get next. We like mandarin dragonets, maybe a chromis, but would like a good fish to swim around with the clown. And I pretty much limited to 3 small fish? I don't want too heavy of a bio-load, and plan to fill it with some corals. Plus something to keep the sand bed shifted..
Do not add another clown! clown fish can be very aggressive towards other clowns if not raised together, and in a smaller tank.. Personally the only damsil that I like is the green chromis. Fire fish would be a good addition, maybe 2-4 since they stay small and don't require large amounts of food. You could also go with bangai or pajama cardinals, long nose hawk, small wrasse, scooter blenny or algae blenny. There are many options of small reef safe fish that grow slow and don't get too large.
Actually, I just reread the whole post, since I didn't remember from over a year ago. I think he said put a coral beauty in there, on the advice of his LFS !) Can't imagine a coral beauty in a 20 gal, so I wonder how it's doing.