I have a hang on the back with media from my 220 and a big sponge filter.Looks like a bad case of drowsy and secondary infection
1 teaspoon of epsom salt per 10 gallons and use of maracyn 2 and/or tetracycline would be best bets
What armrest you using for filtration?

If i were you i wouldn’t go back to that lfs ever again.. my lfs wont sell sick fish (he goes so far as to turning lights off tanks where sick fish are so you know you can’t but from them) and has quarantine tanks if he sees fish sick to separate them from the rest of livestock. However if little guy is still eating, there’s still a chance. Once they stop, imhe, it’s the end.