I'm assuming this applies to tanks that aren't running chillers...which would also make the room absolutely awful during a heat wave but the tank itself might be fine lol.
Things I've done and am doing: open things up as soon as it gets cooler out than in, button the house up as soon as it's otherwise in the morning. Lights off, in the home and tanks. Lighting is a big heat source on some tanks; they can be dark for a brief bit or at least during the worst part of a heat wave. Cover windows from the outside if possible (often it's not, but it does make a difference). Close doors to parts of the house that get warm and open doors to places that stay cool. Go open top or put mesh over the top if you have jumpers. You can use something like small-spaced bird netting and tape in a pinch. Also, unplug your heaters! Sounds dumb and you might think they're not running when it's sufficiently hot to be above what you have them set to, but I have seen otherwise with a good many of the commonly available brands that will still flick on for a bit here and there when there is absolutely no reason for them to be doing so.
The ice bottle method may be controversial but I have used it in a couple of heat waves in the past to keep my fish from starting to lose it and drift around. All the stuff I wrote above won't do a darned thing in long heat waves when your place is worse than the outdoor temperature, has broken AC, not enough ventilation to even cool down to outside temp, and is unable to sufficiently dump heat at night. In that sort of horrible mess where it is impossible to cool a room, I have used half filled soda bottles of ice. It is possible to use them in a way that keeps the temperature pretty stable without rapid/large swings. A single 20gal is easy to moderate with a freezer full of small soda bottles of ice. However, rotating them to re-freeze will also stress your freezer and make more heat from that - so you cool your tank at the expense of dumping heat into whatever room the fridge is in. Cooling the room when possible is always a better option but sometimes it's just not an option.
Also I hate to say it but the best defense against sudden heat is...a well insulated house with thick walls and skimpy on the windows. Obviously that's hardly something you can run out and pick up at the store in an emergency, but it's worth thinking about for the long term if these events become more common. It was something I looked for when I moved a year and a half ago.
