If you have MH lighting, shut it off; if you have T5s, turn the daylights off and just leave the actinics on for now. If you have a sump enclosed under the tank, open the doors to it and get a fan blowing towards the surface of the sump. Otherwise or in addition get a fan running across the surface of the tank. You'll be evaporating more water but that is what will help cool the tank.
Also if you have multiple pumps in the sump for UV, carbon or GFO shut them down.