yep, you need a heater. you know, this may sound weird, but you could try an undertank heater. I have one, and it works really well. you don't need to put it in the water, so it doesn't detract from the asthetic appeal, and they circulate heat much better. the one I use is a 4 watt zoo-med heater, and it keeps my tank at exactly 77 degrees F. they're avalible at your local LPS for only $15, maybe less.