I'm cycling a 100-gal with a 200-watt Won in it right now, and so far it's kept the tank spot-on at 79 deg. I have another 200-watt that is going in the sump as soon as I have the last shipment of rock in and can determine the final water level and start the overlow, but it looks like I could get by with the single heater if I wanted to and not have any temp issues. I'd definitely recommend them.