I had a 20 gallon for awhile and had, in addition to many corals, a little percula clownfish and a damsel. I didn't like the damsel and eventually found a new home for him (too aggressive), but the percula did great. I wouldn't recommend much more than like 2 really small fish, especially if you're going to do a reef system. It's easier to have fluctuations in water quality and the corals show the fluctuations quickly. I had really good luck with leather corals, button polyps, mushrooms, and a smaller anemone. All of my snails (turbo) and hermit crabs helped with the algae control. With a small tank I recommend that you have PATIENCE and start slow. Don't fill the tank up with stuff quickly.