plenty of small fish that would work like little gobies, cardinals, anemonefish, small hawkfish (falcos, or dwarf/pixie are pretty docile IME), etc. I would consider how big the fish will get at full size, but for the most part no matter what I wouldn't put more than 3 fish in there especially if you plan on any sensitive inverts like coral who would do better with a lower bioload. Be careful with starfish like eveyone said. They are sensitive to fluctuations in water quality, temp, salinity, etc.