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My temp in my tank (75g) is constantly changing from about 82 down to 75, is there a problem with my heater?? It is rated for a 100g tank so I'm unsure if I am doing something wrong, or it's just not working right.
What is the wattage on it? You would at least need a 300w heater to work for a 100g tank. 75-82 is definitely fluctuating too much. I run MH and my temp goes from 79-81 consistently. If you are running your central heater, make sure it is not blowing directly on the tank, or in direct sun light.
Also, set your heater to a temp that is least likely to fluctuate. I keep mine at 79, so there is only a 2 degree change.
You could get 2 heaters (150wx2) and set them both to the same temp. Yours may end up having a problem, and having the second one will help minimize problems if it ends up sticking. HTH