Sorry about your guppy
Hope you're able to treat and save the others, bad bacterial infections like that can spread like wildfire and wipe out a whole tank
For the future, you don't need to spend a fortune to have a quarantine tank. You can look for a bargain second hand set up, which will often come with lights, filter and heater, and you only want a small tank if you're only keeping small fish like guppies, a ten gallon is perfect.
Alternatively, you can even use a plastic storage tote like this;
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As a temporary quarantine tank, only using it when needed. Get a cheap sponge filter and duel output airpump, run the cheap sponge filter in your main tank in addition to your usual filter until you need it for quarantine, so that it's cycled and you only need to transfer it to the tub, and a cheap heater with a heater guard so it doesn't melt the plastic. The duel output airpump means you can run the sponge filter and an additional airstone should you need it. Sponge filters provide decent aeration alone, but should you need to medicate or raise the temp to treat something like ich, the additional aeration from an airstone is always good.
Ta da! You have a cheap quarantine set up that you don't have to have up and running all the time, only when you need it
Believe me, I understand being on a budget, and that this hobby smacks you constantly with more and more expenses. But it also doesn't hurt to have a 'spare' filter and heater when an emergency happens, because heaters and filters have a habit of breaking at the most inconvenient times... like in the middle of the night in winter (when my last heater broke).
It's also much cheaper and less heartbreaking to treat new fish in quarantine, than to have an outbreak in your main tank, where you'll likely have to spend more on medication to treat a bigger volume of water than the little ten gallon quarantine tank. I also learned the hard way that livebearers like guppies are often exposed to a lot of parasites and illnesses, as they're mass produced in fish farms abroad. Mine appeared to be fine for months, and only later was it clear they were carrying worms. Since I share equipment like syphons and buckets between all my tanks, it meant all tanks had to be treated. It cost me a lot more in medication to treat 87 total gallons than it would have to treat my 12 gallon quarantine tank!
Fish stores are also exposed to all kinds of diseases. Think of how many fish they're bringing in from all over the world, many of them very stressed from being caught and shipped, and many stores have a shared filtration system, so those diseases and parasites are carried along to most of the tanks. You don't want to pour water from the fish store into your tank, or put fish store fish straight into your main tank without quarantining them first. It's just not worth the risk.