I'm guessing a chiller might be in order. I could not keep my 75 cool enough, even with the AC
set at 78. Got one on Craigslist for a decent price.
It is not good to have more than a 2degree fluctuation in a day. The frozen water bottles will bring temps down, but will also bring wild temp swings. I am no expert, but that has gotta stress the fish to some degree.
Got my chiller and no more worries. Can keep the AC
up a little higher while at work and the savings will eventually pay for the chiller.