Thank you for clarifying the timeline! Part of this whole thing is that we don't really know what you've done/what you haven't done. I honestly thought you'd been doing a lot more than that, because I know there's been some take out your decor/don't take out your decor/use your filter/don't use your filter.
Honestly, I'm not trying to say you did anything to him on purpose. The PH down just seems like a very likely culprit in his newest issues, and I was only trying to let you know what it can do :/. There's no way you could have guessed, and you are right that it's MUCH better than a dead fish.
You can probably keep using it, honestly. Just next time... don't do it suddenly. PH changes are one of the most stressful things for a fish. If you have a cup/bag to float him in, next time you change the water put him in that and float it in the tank once you have the PH where you want it. You might have to float him for up to an hour or two depending on how much you're changing the PH (really with as drastic a change as over a point, you'd have to float him for a while...). Every ten-fifteen minutes or so add a little bit of tank water to the water in the cup/bag. This will acclimate him to any chemical changes slowly.
Basically, if you want to use the chemical... go ahead, but don't add it directly to water that Guppy is currently in!
As for the filter, I really have nothing against them. I would still be using mine, except Indy was trying to jump in the baffle and tore his fins
. I'm pretty sure ich bacteria is nigh impossible to get rid of completely from water, so I don't think it matters in that regard? As long as his immune response is fine he shouldn't get it again. Stress Coat is a great idea, because it replenishes his slime coat, his natural defense against ich.
The one thing the filter /will/ do, though, is start cycling the tank. So if you want a cycled tank, read all the guides on here about it. It's really difficult to hold a cycle in a 2.5. The filter will also help keep the water clean, but... I think it's mostly a matter of preference.
Pure distilled water /is/ bad for your fish because it has no buffers against chemical changes. However, mixed with tap water it is fine for your fish. If your water is always naturally 8.8, though, it won't do very much in terms of lowering the PH, I don't think :/. 8.8 is really super high for tap water! You can try it, though. Basically fill your tank up with a mix of 25%-50% distilled water and 50%-75% tap water.
You haven't made him blind! Basically ions were restricted from flowing in his eyes from the sudden change. Given time he should adapt and become normal again. It was the suddenness of the drop, not the change itself, that would have done this. That's why you have to do it slowly - otherwise that'll happen. He should go back to normal, it'll take a while though.
You're just trying to do your best by your fish, I know. So far you've done a great job of taking care of him, and I'm sure he appreciates it.
Yeah, ammonia will still show on tests when you use Prime. It's been converted into another chemical that isn't toxic, though.
Anhel wasn't saying you shouldn't change the water for 3 days, they were saying you should. Definitely do! Clean water is one of the best things for him now. I would say just acclimate him slowly and carefully every time you do a water change from now on... and hopefully that'll help.