goldfish are pretty easy, they can withstand almost anything. i've got a 16-gallon. the tank itself is about $30 and the filter is about $30. one of the problems that i've had is that if the power goes out for just a little bit, the filter loses it's prime and most of the fish die. but yeah, don't worry about using tap water, you will get some of that water stabilizer with a whole tank setup, and it'll be fine. I have tropical fish now. the pet store people always tell me they'll grow a bunch and not to have too many in the tank, but none have grown or just die, so you can fit a lot more in there. the neon tetras are my favourites; they're pretty, small (to have a bunch instead of a few big fish in the tank), and they're very cheap. I just got some glowfish and didn't realize they were $6 each. ouch. otherwise you'll pay about $2 for each tetra. i feed mine every other day and have a couple snails in there to clean up the gunk in the tank. another thing i've found is not to turn your filter intake up too strong. it's down now, but those mutant tropical fish can barely swim and i'd regularly find them stuck dead to the intake. but otherwise it's not really that expensive to have fish as long as you don't buy the expensive kinds. it's limiting on the niceness of the fish, but whatever. or just go goldfish--they take any punnishment and live forever. also at a lot of pet stores i've looked for labels according to how delicate the fish are, whether they're "beginner" or "expert" fish, for just how perfect you need to maintain the conditions. but otherwise tanks are pretty nice. they're really nice and visitors like to look at your fish, and they're not like real pets that you can get one of those 7-day feeders and take a vacation and not worry about them dying on you.