Putting the Tstat probe IN the tank isn't "wrong," exactly. Not that Serpentmerchant said it was... The best reason for ME to put it outside is for cleaning purposes. I have my probe in the tank, and it's a pain to clean around it. It's secured to the bottom glass to keep it from losing contact with the floor. If you keep yours inside, secure it with silicone adhesive or hot glue. Just don't cover the actual probe tip.

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