Pending on what you plan on using for a filtration system, I would say maybe 4-8 inches of fish, you might be able to have more if you have live rock in the tank, but you do not want to over crowd your tank. 30g. tanks are good for small fish such as pseudochromis and other fish that stay a small size. You do not want to crame fish into that small of a tank, it will just end up giving you nothing but problems.