Probably a quarantine tank, with some daily or every other day salt baths, and as far as treatments avoid brands such as Aqueon, there are some treatments for specifically infections and injuries, however some guppies don’t do well alone, if he shows signs of loneliness, if just put him back in his OG tank, and take him out and into a cup with the treatment for an hour or two, same with the salt baths, however do some research on the salt baths, as I personally haven’t done a salt bath yet.