Your best bet would probably be to simply put a small space heater in the room that the tank is in. Personally, I have a small electric one for the couple of weeks during the winter where it gets really cold even though temps never come close to where the darts can't handle it.
I have same problem-my frog room is on cold end of house. During cold months day temps are ok with all the lights and house thermostat set on 74 degrees, but at night with house thermostat on 68 deg.and no warmth from lights i worry. So i bought an oscillating room/space heater that has a built in thermostat-i set therm on 70 degrees and all is good! I bought heater at HD for around 50 bucks.