I'm so sorry, I missed this earlier, thank you for the reminder!
Which test kit are you using? Because the results aren't adding up I'm afraid.
Have you seen any of the fish doing white, stringy poops?
I ask because i suspect he's likely carrying worms. Bloating like that usually indicates either worms, a bacterial infection, or organ failure. The fact he's still alive suggests it's likely worms, and it's incredibly common for livebearers like guppies to be carrying worms when you get them - I went through it too! If it is, then you'll need specific meds, so don't rush off and buy a generic wormer just yet... answer those questions and read the thread I've linked below first, which says the active ingredients needed to kill worms.
Now disclaimer: I am not an authority! On fish disease, or anything else for that matter! This is just my suspicion and best guess. It's tricky to diagnose illness in fish and none of us are aquatic vets, but it's not really reasonable to take a £3 guppy to a vet and pay for expensive lab tests, and a non-specialised vet likely wouldn't know either. So we have to flail around a bit in this hobby, and do the best we cna based on research, personal experience, and eliminating the most obvious suspects first.
For first aid, especially since you haven't been doing water changes too regularly, please step those up. At first, change 20-25% of the total water volume daily, making sure to give the substrate a good clean at the same time, and get your hands on an API Freshwater Master test kit- the liquid testing kit, not the dip strips. It might seem expensive, but it's much more accurate, and we need accuracy when testing those numbers, and dip strips just aren't as reliable as liquid test kits. You also get a lot more tests out of the kit, dip strips wind up more expensive per test. My API master test kit has lasted more than two years, and I have 3-4 tanks
Once you've done those daily for 2-3 days, increase the volume so you're changing 50, 60, then 70% of the water for each daily change. This won't be forever, it's to improve the water quality which boosts fishes immune system, and helps them battle any illness they're fighting. Try to feed the fish up by feeding little and often, a variety of foods if possible - without over feeding the tank and leaving uneaten food in there. Would be best to give the main meal shortly before the daily water change so you can clean up any uneaten food. But the small daily feedings and variety of foods should help give them enough nutrition so the worms aren't taking all of it.
After this water change schedule and after you've treated them for worms, you want to aim for a routine of changing 50-70% of the water a week, usually. That really depends on how heavily stocked the tank is, and whether it has a lot of live plants or not, but you can use the accurate water tests to determine if 50-70% is enough for your tank, or whether you need to do more. But can work that out later. For now, the daily changes, starting slower/smaller volume changes since you've been doing smaller less regular water changes and you need to give them time to adjust to changing parameters.
Here's the thread with more info about worms, and which meds are effective. I'd treat for both round and flat worms, since they can carry both.https
/www.fishforums.net/threads/what-to-do-if-your-fish-has-stringy-white-poop.466276/