Aquarium Heater Issue

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Registered Member
Dec 12, 2016
So yesterday I bought the aquatop 100 w heater for my 20 gallon long aquarium. I followed the instruction manual and also placed the heater near air stone so it has circulation. So what is happening is the heater, at least from what I understand so far, is that I'll turn the heater on let's say at 76 degrees, the heater will run past 76 to the 80s, I'll slowly turn the nob down and wait and the heater won't shut off. So now with the water at like 82 I unplugged it for about 30 minutes to let it re-accumulate to the water temp, I plugged it in set it to about the desired temp, it didn't kick on so I fiugured it would when the temp lowered. Anyway I came home later and the water was at 72 and the heater wasn't on. Any suggestions on what to do to try to set the heater or should I try taking it back. Thank You


AC Members
Jun 28, 2006
SF Bay area, CA
You might take it back, that sounds like big temp swings. But...keep in mind the heater temp is kind of relative to its "set point"; many heaters & thermometers are not calibrated accurately. You may have to set it to, say, 74F to keep it at 76 (or 80f to keep it 76). Cheap thermometers are not very accurate either IME.

So, if, after you play with settings (very small adjustments) for several days & it still seems unreliable keeping a temp, return it & try again. But there may always be discrepancies, not a big deal if you can figure it's "happy setting" out...before your fish get cooked.

I have no experience with your particular heater but almost all can be like that. Fish should be ok within a few degrees...what fish do you have?