heater not working

[peaches3221]

I have a marina 10 watt heater in my 1.5 gallon critter keeper. it says "up to 2.65 gallons" and preset at 78° F but the thermometer says 72° F. i thought maybe the heater was broken so i unplugged it and checked the temp a few hours later and it went down to about 66° F. I guess my room is too cold for the heater, cause the instructions said it only works within 9° F. tried turning the temp up in my room but it's drafty. and it's even colder downstairs. so should i get a 20 watt heater? or, I have a little room heater fan thingy, should i just try to warm up the room to about 70° F?