Okay, so yesterday I was wandering through Goodwill and came across a pretty cool-looking mini-aquarium, and after making sure the light worked I bought it, figuring that surely there was some fish that could live happily inside it. It's lighted, but not filtered or heated, and holds about five cups of water. But are there? At first, I was thinking a tetra or two, but then I read up on them and they don't do well in groups of one or two. Are there any fish that could live happily in an aquarium that size, or should I invest in robotic fish? Also, what would the maintenance be like on a tank that size? How often should the water be cleaned?