I think it would take a little more then 90 degrees to boil a fish. At this temp they would become very sluggish and gasp at the surface for air, but they wouldn't actually "boil". It would take a temperature of atleast 100+ degrees for that to happen. They will actually explode.
Ummmm, I think this statement really requires some proof. Pure water does not boil until 212 deg F (100 deg C). So, air temperature would not cause boiling. In addition, any impurities in the water would cause it to boil at higher than 212 (part of the reason to put salt in your cooking water). I don't have any real comment about 'exploding' though you are free to call me skeptical. Of course, your fish will not live up to 212, but as for them boiling... highly unlikely.
Now, there are several ways to cool a tank. If it is a little warm, opening the lid/hood and running a fan over it will help. The evaporation cools the tank (think sweat). Similarly, you can wrap the tank in wet towels and run the fan on it. As the towels dry, again, the water evaporates away from the towel which will cool the tank.
An airpump will help, but the main thing is that warmer water holds less gas than cooler water, and coupled with that at the higher temps the fishes' metabolism will be running faster so the fish requires more oxygen. Mainly, you just want to observe. You can always perform water changes with a few degrees cooler water. (just a few degrees, sharp temperature changes are bad for their health) Ice can be used in an emergency, but again, sharp temperature changes... If it becomes a chronic problem a chiller may be needed, whether you buy one or DIY is up to you.
