Ya, that is a parasite of some sort. If one fish has them, its likely all the other fish do as well. However, this is the one medication that its safe to treat the DT with. It comes in little food pellets. He will probably be able to deal with the parasite on his own, almost all wild caught fish have them. If you do decide to treat, you are going to need to feed the medicated pellets for 2-3 days once a week for a couple of months to get rid of the infection, otherwise they will just be re-infected.