What are the dimensions? A coral beauty would be fine in a 37 most likely. I used to have a potters in a 40 who did great for 3 years until I broke the tank down and gave him to a friend. Other than that you could do clowns, a royal gramma, blennies, gobies, cardinal fish, small wrasses, small hawk fish.
20 Gallon SPS reef
29 Gallon FW "fat catfish" tank
20 Gallon Long Dart Frogs "Zig" and "Zag"