Synirr
"No one is a failure unless you try"
I don't normally post in here, but I saw the topic title on the main page and thought I'd add my two cents. Assuming it's between 3 and 5 gallons, I'd go with either the aforementioned betta, or a dwarf puffer. Most people recommend 5 gallons minimum for a dwarf puffer, but honestly, mine doesn't use her full 5 gallons and I think DPs can do ok in a smaller tank as long as you keep up with your water changes. They are sensitive to water quality though, so that is an absolute MUST. I now have a pair of females in a 5 gallon tank together, they're great fish.