I'm not sure exactly where to post this, so mods redirect at your discretion.
This is just an FYI for those facing a similar problem, I thought I'd share my results.
I live in a condo and even with the heat turned off in the winter it's still 75F, which is somewhat convenient because that's exactly where I like it.
Unfortunately this led
to a tank temperature of about 82F with no heater. I was already using an aerator, and was struggling to figure out how to lower the temperature. I raised the lights slightly and reduced the time they were on, but still couldn't get the temp low enough. Even with no lighting on at all it would peak around 78F during the day.
My 75G tank is glass covered with plastic tabs on the rear of the glass cut out just enough to allow the pipes and wires in (to avoid escaping fish).
So I did some more research and installed a pair of small pentium-2 CPU fans (about 2" wide) which fit perfectly in the plastic at the rear of the glass. Turned them on, and after just 8 hours
the temp had dropped from 83F to 76F. After leaving it on overnight the following morning the temp was 68F. So a small pair of fans dropped the temp by a whopping 15F in 24 hours
They were, however, loud, so I replaced them with a 4" 'silent' single fan and only have it running from 3pm to 11pm (lights on at 11am and off at 11pm) and it keeps the temp in a much nicer range of about 75F to 78F with a peak matching what you'd see outdoors.
The fan and power supply I already had, and it costs about 6c/mo in electricity to operate and helps to humidify the condo.