I think that people are hitting on the right path. The only way that 80 would be killing your fish is that if your are having a swing of say 75 during the evening and 80 during the day. Idealy you don't want more than a swing of 3 degrees either way. 77 maybe ideal but you may easily run 80 or even a touch higher (I don't ever suggest getting above 83, although hitting 83 won't immediately kill a fish).

If this is on a regular basis your fish will easily adjust to 80 without any ill effects. Those of us with hotter weather do have to consider chillers. If you don't want to go that route but want to cool the water slightly I would suggest checking how far your lights are off the water and incorporating fans into the system. Your house temp should allow you to help control your temp better than dropping $$$ into a chiller. Try this first would be my suggestion.

Now for the dying of your fishies, check your paramiters and determine that none of those are off first. That would be where I would look not to the temp. If you are having a lot of die off you may be experiencing swings in ammonia and nitrite. These are both very deadly to fish and corals alike. Check those are see what they are. If not off report back and we maybe able to figure something else out for you. Post anything that you can thing of. It may just be something simple that you have overlooked.

Angie