where is all this heat coming from?

Are your lights covering much of the tanks top surface area ? if so they could be cutting down ventilation much like a cover.Raising them a couple inches could be a simple solution.My tank is 44gal. lit by 250mh about 8in. above water level and Ihave no problems as long as my room temp is below 78 degrees F,no fans except in the light itself.

 

Thanks to everyone for the replies. My heater is set for 76 degrees, but I guess I will turn it off for a while to be certain that it is not contributing to the problem. My PC light does generate heat, but it is elevated about 2 inches from the tank by the mounting legs. My T5s are fan cooled. I'll try turning off the heater and maybe mounting a fan on top of the tank. Will keep you posted.