Aquariums: Fish with white stuff around its tail?

My fish just died a few weeks ago due to an infection of something unknown that ive been trying to figure out. I've been feeling so guilty and i need to know what this is. It all started out when he got rily red on his tail over night then the day after it all got covered in that white layer which completely destroyed his tail. It looked like it was beyond repair then i looked it up and it said it was js healing so i disregarded it but then he died a few days ago and I couldn't stop crying (i hate seeing animals die)
PLEASE HELP ME FIGURE OUT WHAT HAPPENED WITH HIM I HAVE BEEN FEELING SO GUILTY : ( when he was still alive he could barely swim due to his whole tail being on that white layer. It was heartbreaking. I will never get over this fish if I don’t know what was the cause. I feel so bad because he probably died in pain due to that white layer 🙁 please help