My Beta fish has been sick and im not 100% sure why?

So my Beta fish that I got about two weeks ago is sick, and I’m not entirely sure with what. his first symptom was that his underbelly and chin area turned white, when that happened I started treating him for a bacterial infection but then his right eye started to look "scratched" like something had scratched the outside of his eye and he was going blind. Then his third symptom was he got fin rot. His back fin seemed to disappear just over night. So I’ve been treating him for fin rot and bacterial infection. I tested his water today to see if there is anything out of balance and his levels of everything are fine. And I’m just wondering if you have any idea what could possibly wrong with him. I keep his temperature constant, and above 80. He has no real plants in his aquarium, but he does have a snail in there.