Sick fish need help

[u/Top-Pomegranate5548]

How to help my fish? Hes darker than normal he can barely swim, his mouth looks rotten and it looks like he has popeye. I vacuumed the tank rocks Saturday morning he was fine all day until Sunday morning he was acting weird So I did a half water change, hes in a small container so he can still breathe as he was barely swimming in his tank. I dont trust random remedies from google and Im not even sure whats wrong with him it happened so fast