I've been having the same problem with my 10 gallon mini-reef. Hot days, PC lights, no AC ... hot fish/corals.
What seems to work for me, although its a PIA.... I had 2 gallons of aged seawater which I stuck in the refrigerator. I took 2 gallons out of the tank, and slow dripped the 2 from the refrigerator in - keep going back and forth every couple hours on a really hot day. Lot of work, but I'm home for the summer.
Also - other people have recommended freezing seawater in ice cube trays, then adding them, or using the small lunchbox ice packs (enclosed in ziplock bags so no leakage problems).
Maybe one of these might work ... :notsure:
Anyway - September's here, so it shouldn't be a problem too much longer (depending on where you live).
Good luck!