I would not say these fish are skinny! Fish in the wild are more prone to be looking like this in my experience.
IMO flakes are a waste with fish this size, they should be fed pellets of a 1.5-2 mm size.
If you are trying to put some weight on them try feeding them twice a day maybe 3 times a week, other than the normal once a day.
From what I see now the fish DO NOT need to be treated!
BUT, Prazipro would be the way to go as far as treatment, but check out a few sites for sickness and cures, as in some cases if not treated properly the problem would re-occur.
Cheers,Larry