Both my tanks got up to maybe 84 when we had those wickedly hot days recently. I do not have AC in my old apartment, so I managed this by leaving the lights off 24/7, feeding less, lowering the water level for more surface agitation below the filters, rotating frozen bottles of water through the tanks, cracking the glass lids on top of the tank, and running air pumps with air stones.
This worked for me because the increase in temp was gradual (again, no AC so it stays on the warm side anyways). Had it jumped from say 70 to 84 quickly, I have no doubts there would have been a loss or two. All my fish (4 sunfish, some Odessa barbs, a Clown pleco and 4 yellow bullhead babies) made it through with no problems. No gasping at the surface or anything like that.
I would certainly prefer cooler temps for them (and us) but the combination of things I listed below managed the heat enough to keep everyone comfortable.