I was just about to start a similar topic.
It was 30c (83f) here yesterday, 25c today, and my tanks have been getting really hot. My big tank got up to 29c (82f) yesterday which can't be healthy for the fish.
I've got all the windows open, tanks are in shade, I unplugged the heaters a couple of days ago and have cut back on the amount of time the lights are on, as this heats up the water too. I've also been freezing big lumps of dechlorinated water and adding them in the filter housing every couple of hours.
A few big lumps lowers the temperature almost a degree c, but it's hard keeping up the supply. All this doesn't seem to have made much difference to be honest. I might have to invest in a portable air conditioner to cool the room down.
Would leaving the hood of the tank open help at all? I've been a bit unsure about doing this in case any of my fish decide to end it all and jump.