I dunno. If it's not too sudden, which it shouldn't be with a decent sized tank, I think a couple of degree swing over 12 hours is OK. If they can stand 86 degrees for an
ICH treatment, shouldn't they be OK heat-wise? I have the same trouble, and I don't think it's contributed to any of my current problems, they were great all summer.
I wonder, if you did want to go the fan route, if a steel screen topper would work to keep them in, but still keep the air moving. Maybe you could freeze the water you use for water changes in cubes or blocks and put the ice on the screen and it'll drip as it melts, pretty cold, but not sudden, and it would keep the air above the tank cooler, too. Hm. That might be more of a long-term solution... it got pretty complicated, didn't it.