A fresh water dip will help but I would instead suggest you QT
the tang and any other fish in the diplay tank. Tubellarian worms are much like ich when it comes to fish. If one has it they all will to some degree. The parasite also multiplies on the substrate or other hard surfaces so once in the tank, it will stay as long as it has a food source. 8O
The same basic recommendations apply as with C. irritans
the fish and treating with hyposalinity will be much less stressful on the fish than a dip but in this case both will work. Although with FW
dips, you may need to do it a few times (which could lead to other issues). Just be sure the fish are not placed back in the display for a full 8 weeks or the cycle will begin again.