My basement can get down to 68°F in the winter (on really cold days 66°F) and I keep my tanks at 78°F. In the winter, my living room is at 70°F, with it going down to 68°F during work hours when nobody should be home.
Every online calculator I use for a 29 gallon tank tells me, in the winter, I will need to use a 150W heater for a 29 gallon tank, and at least 225W for a 55 gallon tank.
I had a 100W heater in the basement 29 gallon tank, and in the winter I struggled to keep the water up to temperature.I guess I could keep 1 100W heater in there in the Spring, summer and fall and only throw a second one in for the winter months.
I know basement 55 gallon NEVER got up to temperature in the winter with a 200W heater in there. I had to throw another 100W heater in there.
The problem with this is price. One 300W heater is a LOT cheaper than 3 100W heaters of any brand. I don't mind putting 2 heaters in a tank. But putting 3 is more than I will deal with.
Speaking of margets...
If I glued some epoxied magnets to the back of the heater, would it mess with the thermostat?