How To Keep Fishtanks Cool In A Heatwave

https://www.practicalfishkeeping.co.uk/features/how-to-cool-your-aquarium-in-a-heatwave/?fbclid=IwAR0FfKwjgNj7YZArvD1wt0hrvDfc_DuFpkY2F-cNzCjQeZzhz9txVVVLL5A

Old article but might be helpful over the next few days. 

Temperature here hasn't been that much different from "a good summer's day", and, with precautions taken (ie blinds down in the fishtank room), tank temperature hasn't actually increased. 

I had a missing fish yesterday evening, who had mysteriously worked her way behind the filter despite all the filter wool and sponge blocking off any access points - but I think that's more due to being chased by an exceptionally persistent juvenile male espei relentlessly pursuing all the females in recent months than anything to do with the heat.  Fish has lost the bottom half of her caudal fin, likely in an effort to release herself from behind the filter, and is still too traumatised this morning to eat breakfast. 

Changing the topic very slightly here but it’s been nice not to see the smart meter reporting what I know full well is lots of fish tank heaters power up simultaneously. I’ve actually just taken a (marine) fish tank down - one of the reasons being the cost of power nowadays. Three tanks and a pond just seemed excessive. I still find myself thinking about moving to cold water/more temperate species too but I can’t escape the lure of tropical fish just yet…

This is a very late response, but having lived in the desert southwest of the USA where it can easily top 38c for days on end, I've some experience with keeping an aquarium cool. As others have done, keeping the blinds closed is a good start. If you have the option, and if it's getting particularly warm in the room, if you can just barely open a window on the side of the house that is in the shade, (or two windows if you can - just an inch or two) you can get at least a little ventilation. A fan blowing across a pan of cool water can lower the "feels like" temp, if you have low humidity, too.

If you are in a humid climate it is harder to keep the temp down, but in a low humidity area changing the top of the aquarium to something like window screening fabric (if a top must be put on it) allows evaporation, which produces some cooling.  The last place I lived in the USA was without air conditioning and we had several weeks of spectacularly hot weather. The tank was small, so what I would do was put one of those blocks that you can put in the freezer repeatedly and then use to keep things cool in a picnic hamper (not sure what they are called) on top of the mesh  over the tank, and covered it all with a towel. The cool air coming off the block did help a bit for several hours. (Note that I did not put this directly into the water.)

Hope this helps someone.