Hopefully we'll have nicer weather for a few days..
Anyway, back when I lived in an above ground floor apartment, I used to keep the room dark during the day (no extra heat from lights) and run the lights at night. I would come home from work on the hottest days and do 30% water changes with cool water. Sometimes I would do another water change a few hours later if it was really hot.
Run the filter output over the top of the water or add an airstone to increase oxygen in the water (oxygen is the first to go in really warm water)
If that doesn't work, move to a basement apartment