During the peak of last summer we had several goldfish that died after several days of hot weather and I suspect it was related. According to the Goldfish article on Wikipedia:
Extremely high temperatures (over 30°C (86°F) can also harm goldfish
The ambient temperature sat over over 40°C (104°F) during the day and only dropped to a little under 30°C (86°F) overnight so it's almost certain the tank temperature would have exceeded that amount.
Doing some research I see there are some commercial tank cooling products available although they are relatively large / expensive and this is only a once or twice a year event in my area. A few thoughts I'd considered are:
Setup a fan pointing across the tank as a form of evaporative cooling. I assume though that would cool the top layer of water more than the bottom and I wasn't sure if the differential in temperature may make things even worse?
Introduce a large amount of tap water that I measured as a bit under 20°C (68°F) because it comes from a cooler mountain range. Considering that would need to be treated quickly chemically I could see that also having potential downsides.
Add ice cubes that have already been treated, but I imagine that could result in them floating on top and mainly cooling the top portion of the tank.
I wondered if anyone has been in a similar position and can recommend a good alternative or suggest expert opinion on which of the above options may be best?