Help with betta fin rot!! White fuzz

[Fin35]

I have a betta in a 10 gallon. I previously had him with Ember tetras, when I found out they were nipping his fins I found them a new home. His fins never totally healed bir it’s been 9 months and he seemed fine. Now he has white fuzz on his top fin and his body has two discolored spots. The aquarium store said to feed him Kanoplex mixed with his food because I have plants and snails. I mixed it and fed it to him tonight. I accidentally put a bit to much because of using a spoon, if it sinks and the mystery snail gets it, will it kill her? How long should I feed this to my fish and how often should I do water changes? I also have Indian almost leaves should I do his with snails? I really wanna put a ton in but I know it might lower the Ph to much! I need to save my fish