I am also having a problem with a Zoo Med small sized (10 to 20 gal.) under tank heater. I believe my problem is that im using aspen chips as a substrate and also too much of it which is causing it to act as an insulator. As of this writing the temp of what is suppose to be my warm side is only at 76.5 F, which I know is unacceptable. I am going to switch to newspaper and believe that is going to solve my problem. What type of substrate are you using?
If you are sure that the temperature has changed, and that the change has been signifigant, I would buy another brand UTH. I was considering this, however I realised soon after that the wattage on most UTHs are the same (around 8 watts) so I decided against that route.
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