Okay... So I have a tank that is 5/8th of a gallon (still empty... awaiting sea monkeys). I didnt read the "instructions on how to keep fish" when I got it, but today while I was bored, I decided to look at it.... it actually says that you can keep goldfish in it, and you only have to replace half of the water once a month
Just thought I would share this... (almost as bad as the time when i picked up a 2 gal tank for my baby guppies, and there was a picture of a large ornanda goldfish chillin in there with looks like 5 or 6 tetras)