I have very successfully used a boat dehumidifier for 10 years in my rolling roof observatory.
. Fantastical interpretations of my perfect spelling courtesy of Apple.
NEVER USE AQUARIUM HEATERS OUTSIDE OF WATER!!!
Not sure about an aquarium heater....wouldn't that need to sit in water? I have used a gun safe heater to keep areas under cover warmer and fight condensation. They are pretty cheap (around $25) and are simply a 1' piece of aluminum tubing about 1" in diameter with a heating element inside. There is no adjustment for heat but the standard setting works fine. Bear in mind if you have this or any heat source in the vicinity of the telescope the heat currents may mess up your seeing.