sorry it's a bit hard for me to describe what the problem is. To simplify everything, the heating pad is just heating it to random temperatures. I cannot control it with the thermostat because it will just do whatever. It isn't from a faulty heating pad because I've already bought 3.
I looked over everything that could possibly be wrong and what I'm seeing is that when the heating pad is first plugged in, it eventually heats up to a good temperature and the hot spot is perfect. After a while, the tank just isn't getting heated up. The heat pad is still working but not giving enough heat to the tank even when I turn it up higher. The only possible thing I can think of is that the thermostat turning the heating pad off is making the tank cool down quicker than it's being heated.