if you want tropicals, the smallest you can get are otocinclus catfish of the family loriicaridae. they are social fish a will wither if not kept in at least pairs, or trios. they need heaters. they are sensitive and eat almost exclusively algae, so make sure your tank is well aged and chok full of algae before you buy one. cories are also small, and are opportunistic scavengers and will eat just about anything. they are schooling fish and must be kept in groups of at least 5. need heater in most cases, except for peppered cories. for small coldwater catfish, madtoms only get a few inches in lenght, but are rarely seen in stores, and if you have your heart set on one you could try to catch one as long as you live in the midwesten or easten USA.