Ok, so, I've had my tank heater running since about 8 p.m last night. According to the thermometer, it's about 75-77 in the tank right now, after being run for almost 24 hours. the other day, Gerty was sitting at prime 82 and ther heater had only been on about 7 hours. (I turn the heater off when I can't monitor things.)

So either:
A. My tank does NOT hold heat. At all. (if this is the case, I need a remedy for it.)
B. That thermometer sucks.
C. The heater was a waste of 30 bucks. (It's a Zoo-Med stick on tank heater.)

How do I figure out which issue is the problem (if it's not all of them.)