Marina 200w heater

[rfn379]

I have one of these for my 40g breeder Mbuna tank and it seems very inconsistent. I set it to 79 and it seemed to work fine at first. Then the temperature spiked up to 86 on my probe thermometer (even got a second one to confirm the right temp). I lowered the heater setting to 75 and the water still shows a temp of about 81-82. Is 200w too much power for this size tank? I am new to keeping African cichlids...which is why I joined here...and would like to resolve this before my fish get cooked. No matter what I do I just can't seem to get the temp setting just right and it seems extremely unpredictable. The room is normal room temp so I don't think that is causing the problem so I am kinda stumped here. Any help would be great. Thanks.

 

[Phranque]

This is the exact reason I use a separate digital heater controller for each tank..... Maintains within 0.7F