You could try installing fans to blow over the top of water. It should help bring it down a few degrees. That is what I have to do in late summer. Like you, I don't want to spend $$$ on a chiller either.
FWIW, one of my older tanks used to reach 84/85 all the time in the summer. I didn't have a fan, the lights were enclosed in a canopy and a swing of 4 or 5 degrees a day was a common occurance. Anyways, this was a mixed reef and neither the fish or the corals seemed to mind. GL.