we don't have AC so on the occasional really hot day I will turn all/most of the lights off, put a spacer in the lids so they close with about a half-inch gap for improved airflow (closed enough for jumpy discus) and I also drop the water level a little so there's more surface agitation to both release the heat and increase oxygen in the water (higher temp water holds less oxygen). If the hot spell is going to last awhile, then I will shorten the lighting period and set the lights so they're on in the middle of the night when it's coolest, and off during the day when it's hot. If it's really, really, really hot then I will also freeze a 2-liter of water and let the 2-liter float in the tank to cool it down, and maybe drop the water a little more or dribble in some hydrogen peroxide for a oxygen boost. That seems to work for me really well, and luckily I have south american fish that can handle high temps pretty well. During the summer the temps are typically in the mid-80s during the day. Nights always cool off due to the ocean breezes so that helps too.