Keep Cool
The old trick is to put ice cubes in a ziplock bag, seal it real tight with the air left in, and float it in the tank. How many ice cubes depends on how big the tank. I used two ice cubes in one ziplock in my 10 gallon, worked great. For really big tanks, maybe two or three ziplocks.
The ziplock is so ice cubes don't introduce new water or leave cold melty spots in the water. The air is so it floats on the surface and is easy to retrieve when they've melted. In my 10 gallon this did the trick for over 24 hours on a day when the AC failed in this hot climate.