Your tank already has ich, if it's ich. If it's a parasite, hopefully the parasite hasn't ejected itself into the water.
At this point, if the fish does have something, the only way to get rid of the disease in the tank is to get the tang out of there, treat it in another large container for ich OR the parasite, and leave your main tank fallow (fishless) for at least 8 weeks. Otherwise, any fish you introduce to the tank will catch the disease and suffer the same fate during times of stress. If you choose to do nothing, then just try to keep the tang eating, and keep your water parameters as pristine as possible. It won't kill the disease, but it will hopefully help him fight it.