I have a cheap lil 2.5 gallon tank meant for a Betta. I originally bought for my betta but I decided to get him his own 10 gallon home. Its too late to take the 2.5 back and I don't want to trash it. Are there any cold-water species that can live happily in such a small tank? The reason I say cold-water is because a heater would tank up Alot of room.