I bought a trutemp submersible aquarium water heater...100w...It hasn't been true to temp..and I am not sure how to calibrate...the instructions tell me to turn indicator to the temp of the tank right now...my water is at 82 and the indicator arrow only goes to 79...so I set the heater to 77 and put it back in the tank...so it is set at 77 and the tank is at 82 it shouldn't be on from what I understand..however it is on and has been since I put it back in the tank 10 minutes ago.should I take this back or am I not doing something right...I don't like my tank to be at 82..however I am just in 2 weeks of cycle....nothing live in the tank..will this hurt my live rock and live sand...thank you.

