#infection

I cleaned my tank yesterday and did a 25% water change, I noticed last night my Cory had a red blotch on its side and when goggling it suggested to do a salt bath for 15mins and return the Cory back to the tank , done this and this morning the red blotch has gone however still one very unhappy...

i’ve attached some images of my betta fish sweet pea. i have just gotten him a 10 gallon tank, and i was not aware he had anything wrong with him until recently. how do i treat him pls help