It honestly doesn't look like fin rot to me. It looks more like he was either biting it (tail biters are a huge pain in the butt), or that blue plastic plant you have in there with him has been shredding his fins. Betta fins are extremely delicate and it's best to stay away from plastic plants and stick with either fabric or real. I'm not sure what you mean by water "cleaner" (conditioner maybe?), but no it wouldn't cause that damage to his fins.
I'd stop with the pH down. Unless you have acidic water or unstable pH, just leave it. Messing with the pH is extremely bad for the fish since it's hard to always get it constant. Huge pH fluctuations too quickly send fish into shock. My bettas are on my water with nothing added and my pH is around 7.8, so you're perfectly fine there.
I admit that I didn't read through the whole thread, but I did see that you have a 1.5 gallon tank and saw the picture of your filter. You won't be able to cycle that tank so you will need to do frequent 100% water changes. I'd say about every 3-5 days, any longer and you will risk exposing your boy to ammonia. Dirty water causes sick fish, shortened lifespans, and possibly death. It's also what causes fin rot, but that doesn't look to be your problem at all. Just keep his water extra clean, remove sharp decorations, and everything should be fine (if he's not a tail biter that is).