hi bro.
what brand is your heater.
some heater need to be calibrated before use.
different country different standard.
My Jager Eheim 300W.
says pre calibrated.
i put into water. set at 28.
water actually is 31.
luckily didnt set at 31. else i dont know how now.
there are a few dials on the top there to calibrate. so really need to take note these small small things.
RIP you fishies.
Condolences
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