Hi. I'm somewhat new to the aquarium world. I've only had a 10 gallon and just recently purchased a 60 gallon. Anyway, I had bought an Aquatop 300w glass submersible heater and I followed box instructions and it has never worked properly. I've had it about two weeks, the temp is set at 79F and my thermometer reads 68F. Luckily all I have in the tank right now is four zebra danios and they don't seem to mind the temp. Does anyone have any suggestions as to what heater would be best to replace my crappy one with?