Can't get temperature right

I cannot figure out why I cannot keep the right temperature in my 29 gallon. I want to keep it between 80-81 F. I had a heater with no temperature labels on it which was a pain. Basically just trial and error to try to get the right temp. So today, I went out and bought a 150 watt Marineland heater (says that it heats up to 40 gallons). I set it to 80....and when I checked back later it was pushing 83.

So I have been slowly moving it down. I am now at 78 F and my digital thermometer is reading 82.2 F on the side of the tank farthest from the heater. I am at a loss and I hate to keep changing the temperature back and forth with fish in my tank. Any thoughts? Even when I turned it down to 79 F, the red light went out, and I thought, oh good it will stop heating it for now. But five minutes later it came back on.

 

Bumping this up for you

 

Do these heathers probably need calibrating?

 

How would you calibrate?

 

Now it won't even shut off. I am trying to heat at 86 F for ich, and it is set at 77. The tank is now 88!!! When I turned it down to the lowest setting, it didnt even turn off. I just bought this junk.

 

Same here have a solution

I Had this same problem and marineland heaters are no good. I Had one burn up my fish. My solution is to turn it all the way down. buy a titanium heater if you have money... they are more accurate, and I also need one....

 

I Had this same problem and marineland heaters are no good. I Had one burn up my fish. My solution is to turn it all the way down. buy a titanium heater if you have money... they are more accurate, and I also need one....

I have an eheim jager on the way. I heard good things about it. I hope it is true!!!