It depends on what species of fish you have and how large they will eventually get, you could be overstocked. Check your ammonia, nitrite and nitrate regularly. Readings for ammonia and nitrite should be zero in a healthy system, if not you have a problem with the setup (overstocked, under cleaned etc.) Nitrate can only be removed through water changes at should be as low as possible but most hobbyists I know strive to keep it under 20-40ppm.