Wacky heating between 2 10g tanks

[Jake_the_reefer]

So I have 2 10g tanks that don't have lids or any other means of cooling. They use the exact same brand of heater and are both away from windows in the same room. The one tank is 77° (where I like it) and the other rests at 81° (I want it lower) the heater on either tank arent running because its past the dialed temp. I added cool water to the hot tank to see if the heater would kick on but it didn't and the temp still rose to 81° the only variation between the two tanks is the hotter one doesnt have a protein skimmer. Could the skimmer have a big role in temp?

As a control I used the same thermometer between the tanks

 

[xlLostlx]

Have you tried switching heaters to see if they are calibrated wrong?

 

[Jake_the_reefer]

Have you tried switching heaters to see if they are calibrated

Good point I can try that. It's just odd I set it down to 70 as a test. Added the cold water to drop it to 77 and without the heaters light coming on it rose back to 81 for no reason the other difference between the two is this one is using a much bigger return pump and perhaps that can affect heat. Or maybe biocubes retain heat. I'll play with it and probably mount a fan