Thanks for the input. I keep an aquarium in my basement and with lights on 8 hours a day (6X54 watt T5), that's as warm as the tank gets. I asked because I am planning a 30 gallon clam tank in the same basement with 1X250watt MH and don't expect the tank to be much warmer without a heater. It sounds like the concensus is that I need a heater, right?