If you can't find a heater small enough, just keep the tank in a room with a constant temp. None of my tanks have heaters and they all stay at about 76-78F. Just make sure you get a thermometer of some kind though so you have a way of monitering it. I have the type that stick on the outside of the glass. They don't need filters or airation, but filters cut down on the amount of water changes making them even lower maintenance than they already are. Oh, and don't put them near a window where the sun shines in for most of the day, or air ducts. That could be detremintal to their health. They don't do well with big temp. fluxuations.