For a small tank look for guppies, tetras, mollies, or beta. For a medium tank you should look at cichlids, or barbs.... Cichlids are a very cool and color fish. You can buy them in any color you can ever think of. They can be priced anywhere from 1.00-100.00. They can be found in almost any pet store. Most grow between 3-6 inches, and some such as oscars can grow up to 12 inches. If you are looking for bigger fish that will often live longer you should look at oscars, Bala sharks, silver dollars, red belly pacu's, or tin foil barbs.