My heaters are set for 81-82F. Yesterday tank was 86F when I got home at 10pm. That's on the high side but not lethal. Nonetheless, I reinstalled the acrylic lens in my MH fixture, optimized a household fan directed across the top and plugged into a timer (I love these things
), and blocked off half of my huge sliding glass doors with cardboard. These suckers face SW and turn my place into a greenhouse on hot bright afternoons. Tonight, the tank barely hit 83F.
So if you have alot of window, using shutters, awnings, or carboard (yeah it looks like hell but fish are more important
) can help keep the heat away from the tank in the first place. These really need to be used on the outside, stopping the sunlight from refracting through the glass and transferring heat.