weird heater, adverse effects? (i've finally had enough)

I would start with the cheap fix the 2 dollar in tank heater and use it to check. I normally keep 2 heaters in a tank to make sure they are reading good what can I say paranoid after my first heater fouled and the thermometer didn't warn me it was a few degrees off about 5 and the heater tried to cook my guppies whispers heater that hooked on outside of tank.
 
If you keep blue tilapia they can take soda lake boiling water temps.

I have kept some and the hotter the better.

But otherwise temps around 76 degrees is better.
 
My Rena-Cal is always on mark, except the heat in my dorm is always on for no reason- so my room temperature sometimes is about 85+ degrees. Whenever the room is cooler than 78 (the set mark), the Rena-Cal has been absolutely dead-on.

Thankfully, I haven't seen any adverse effects of the heat, despite my tank temps sometimes soaring up to 88.

I have a digital thermometer with an in-tank probe that's right by my filter intake... Been quite happy with that one.
 
Ditto previous advice to consider purchasing a new heater. For the price of a large pizza you'll be able to find a quality heater that'll deliver a consistant temperature to your tank. I have used Tronics (50w and 200w) and have been completely satisfied. The other brand mentioned frequently is the Ebo-Jagers.
 
I ended up going for the cheapest fix of all: I took the heater out of the tank, and twisted it really hard toward the cooler end. Ended up popping the knob out of where it should be, but the heater still works, and now it maintains a constant 76 degrees. I'll probably still buy a new thermometer tomorrow just to be sure, but as of right now, the heater is working fine.
 
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