You don't say whether your 50g set-up is a PNP set-up like a Red Sea Reefer. Some of these sorts of PNP systems have a hood with T5HO lighting, and an enclosed stand that contains the sump. Under such circumstances, most of the heat transfer from the tank to the surroundings is through the glass walls, and whatever warm, moist air leaks from the hood/stand.
If this describes your set-up, it's possible for the lights alone to heat your tank water significantly. So if you go through the equipment and don't find a problem, you may want to consider what you can do to actively vent the stand/hood, which will speed up evaporation and stabilize the tank temp.