If the fish died it is probably Tuberculosis and the fish has had it for months. The high temperature in your tank sped the process up and the fish suffered major organ failure.
If the place you bought them from is breeding them, then they have TB in their system. However, if they bought the fish in from an importer, the breeder, importer, shop and you now have it in your tanks.
There is no way a shop could know the fish has TB because fish act normally when infected by it, until an organ ruptures and the fish dies within 24-48 hours of showing symptoms. The problem goes back to the breeders who supplied the fish.
Assuming it is TB, you would have to get a vet to do an necropsy and send samples off to be cultured, and that takes a couple of weeks and costs money. Then you would have to find the breeders and confront them with the vet report.
The younger fish may or may not be infected with it. But even if they are infected, the disease probably hasn't progressed as far in them because they are younger and would have been infected more recently. So you will have something to look forward to in the coming months
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The other option is an intestinal protozoan infection, however these usually cause the fish to lose weight while doing stringy white poop and they die within a day to a week or so after they start showing symptoms (stringy white poop).
Again you would need to take a fish to the vet and get it tested, and the supplier could argue the fish developed the infection in your tank.
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At this stage I would not bother doing water changes any more than once a week unless you have an ammonia, nitrite or nitrate issue. I would monitor all your fish because both TB and protozoan infections are highly infectious and can easily be spread from tank to tank via contaminated water, fish, plants, wet net or anything basically that goes from one tank to another.
If the rest of the guppies balloon up and start doing stringy white poop in the next few days to a week, then it is most likely a protozoan infection and you should try Metronidazole. But if no more do stringy white poop and bloat for a month or more then it is TB.
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Make sure you wash your hands and arms with warm soapy water after doing anything in the tanks. And if you have any cuts or scratches on your hands or arms, do not put them in the tanks or let aquarium water get onto the wounds. If this is TB it can infect people through opens wounds in the skin. You don't normally get a big infection but you can develop granulomas (small hard lumps in the skin) where the infection is and it can take months to treat.
If you do develop any sores that don't heal up quickly, or turn lumpy, tell your doctor you have fish and get them to take a swab and have it tested for Mycobacterium (TB). Do not take any anti-biotics until you have the results from the test because if you do get a TB infection, only certain anti-biotics will treat it. Best to know what it is before taking something that might not cure it.
On a brighter note, TB cross contamination from fish to people is uncommon and usually occurs in old people, young children or people with a weakened immune system. And washing hands with warm soapy water after working in a tank will significantly reduce the chance of developing an infection.