Just my opinion,usually stress weakens the fish and chasing it around the tank to catch will stress it more.If the fish is eating I would try to build up his resistance using Vitamin C and some Amino acid with its food. All the dip will do is maybe destroy some parasites externally but what happens when you put the more stressed fish back, right they jump back on. If you are going to put him into an established QT tank(cycled) I would catch him and do otherwise I'd leave him alone. Sorry for the ramble, I just feel the dip will do more harm then good