Hi
So, I've had the Tetra 2-15 Heater since I set up the new 5gal tank! (Word to all: I'd return it if it wasn't too late. Isn't doing wonders for me

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It's the heater where it's preset to supposedly be 78 +/- 2 (plus or minus two)
After being unsatisfactory and only being around 76 degrees and not going higher, it suddenly decides to go berserk and start raising the temp. ALL of a sudden...
Normally it's at 76, but as I check the tank every so often, it keeps rising to 77, 78, and right now currently 78.5
Hopefully it will stop overnight, but I'm just worried in case it won't.
I did a
water change at around 1PM? And now it's about 10:50PM, if that helps with how much it's risen. I don't know if it was a constant rise or anything.
... I really just want a new heater. (But I can't drive. Nor do I have money. ... or supportive parents lol

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What could be wrong? It's completely submerged, and it's only 1month old.
And I don't want cooked bettas in the morning!