Its already been said but i must stress that during these hot spells you MUST have as much aeration as possible to allow enough gas exchange, the warmer the water the less free oxygen it can carry and that is what kills the fish.
Make sure that filter outlets are directed towards the surface and that the filter is positioned as close to the surface as possible, if needed drop the water line a few inches to bring it closer. If the fish are still showing signs of being unhappy (rapid breathing, hanging near the top, stress colours showing) then install additional powerheads or airstones to increase surface agitation.
Keep all lighting off and only switch it on to a bare minimum needed for viewing and try to keep the tanks out of direct sunlight as much as possible.
Make sure that filter outlets are directed towards the surface and that the filter is positioned as close to the surface as possible, if needed drop the water line a few inches to bring it closer. If the fish are still showing signs of being unhappy (rapid breathing, hanging near the top, stress colours showing) then install additional powerheads or airstones to increase surface agitation.
Keep all lighting off and only switch it on to a bare minimum needed for viewing and try to keep the tanks out of direct sunlight as much as possible.
works wonders for my tanks this time of year.....

for the few odd weeks called "summer" here in the UK, it's just not something people generally invest money in.