I have a 55 gallon, open top, acrylic tank in my car garage. The garage is not insulated and it's very poorly ventilated. So far the warmest days have been 102F, which is pretty much how hot it will get here, with a typical min temperature in the summer in the 60s. Before I added fish I setup the tank and tracked outdoor and water temperature for a few weeks.
What I noticed is that the gap between max outdoor and max water temperature widens considerably as the outdoor temperature rises. In other words, it seems that raising the tank 1 degree from 84 to 85 takes a lot more heat than raising it 1 degree from 79 to 80.
I keep my heaters set to 79F. So far the hottest my water has been is 85.9F, even though the ambient in the garage was in the 100s well into the evening hours. Water temperature changes are very gradual and are spread out over 10+ hours.
When I did add fish, I added fish that I knew would do fine in high temperatures and I have a bio-wheel hob filter so I know the water is well oxygenated.
I think it is possible to do this safely for the fish and stress free for you, provided you realize what the risks are and take some precautions; As others mentioned, making sure you have a larger tank (acrylic provides more insulation than glass), that water is well oxygenated and with careful fish specie selection.
I also recommend finding a site that charts your min and max temperatures over a full year, and possibly more.
This is one example:
[URL="http/www.wunderground.com/history/airpor...eq_statename=NA"]http/www.wunderground.com/history/airpor...eq_statename=NA[/URL]