If it is spring disease then the other frogs will almost likely to have the same condition, the yawning that is often seen in frogs is shedding and in a thin frog it certainly comes across as another problem. i would certainly treat all the frogs for parasites and keep the thinner one seperate inorder that he have the chance to build up his weight without haveing to fight for his food. in multiple groups of frogs bullying is commonplace especially if one frog is smaller and not as easily be able to compete for food. maybe ahve a look at another forum one that is dedicated to frogs.
i use this one
talk to the frog
some of the people on this site actually keep the same species of frog you have bought and will be able to advise you better on its basic needs.