Your heater does sound like it's defective. A 50 watt heater in a 10 gallon tank should heat water up at least 10 degrees higher than room temperature if needed.
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Please check out our Betta of the Month contest and cast your vote for your favorite Betta. Click here to see the competition and please vote.
https://www.fishforums.net/threads/...fish-of-the-month-contest-bettas-only.453671/