I agree with V - the Aquaclear isn't sufficient. A good filter needs to have proper mechanical, biological, and chemical filtration. Although your aquaclear has good mechanical filtration (i.e. sucks up all the poo), a HOB filter can't filter a 40 gallon properly because it doesn't have enough surface area for good biological filtration (i.e. for happy bacteria to keep your chemicals in check).
If your tank gets down to 50 degrees, you won't have any luck with fancies. They are goldfish, so they can tolerate cooler temperatures, but fancies are not hardy enough for water that cold. 60 degrees, maybe, is their lower limit. Again, I agree with V - you might have good luck with white clouds, rosy reds, and similar native coldwater species.
You can try investing in a better heater - the newest models are extremely energy-efficient and use about as much power as a lightbulb. Be sure to place it horizontally along the bottom of the tank, so low water level won't cause problems. If you're worried about accidentally unplugging it, buy a heavy-duty extension cord in a short length, and plug the heater in to that. They are usually much harder to unplug, so you won't have to worry about it so much.
Best of luck.