MH and Heat - turning my tank into soup cooking pot?

[WWWD]

Something about your setup sounds weird, I have a 42 cube with a 250 hqi over it, large skimmer and sizable return pump and I am struggling to keep the tank warm.

Is the top of your tank open and gets circulation around it?
Do you have a sump?
How many powerheads do you have in there?
Are there other contributing factors close by that are adding to the ambiant heat in the room, ie; fridge, computer, heat register?

Btw, a 1/10HP chiller will be cycling on and off a lot on a 50 gallon tank with heat issues. A 50 gallon really needs a 1/5 HP.