Tank naturally sitting at 20 degrees

[suitable4frank]

I've recently set up a new 180L tank (been running for 3 days now) and it's naturally sitting at 20 degrees.
I have no idea how to lower it permanently. I can't afford a chiller and I don't really see the point seen as I literally have to lower it by 2-4 degrees.
I've been leaving the lid open to help evaporation: nothing. Turned off all radiators in my room: nothing. Installed air stones to keep up a little water movement: nothing. And turned off all lights in that area: nothing.
My current tank sits at about 17 degrees and it's pretty much right next door to this new one.
Can anyone help at all?

 

[LSuzuki]

Swap thermometers between the tanks. They may just be reading differently.

 

[suitable4frank]

Yeah I have, I even put the thermometer in my tropical tank to make sure it was reading correctly and it seems fine.

 

[michael]

Blow a small fan over the top of the warmer tank. It will drop the temp by evaporative cooling.