This all started after introducing a sohal tang about 2 weeks ago, which was an opportunity we jumped on knowing our new Qt tank room wasn't quite finished. Hindsight... Anyways, what's done is done. Soon after the sohal began scratching against the rocks now and then but seemed healthy otherwise.
Then about 5 days ago these dots/lines appeared on the sailfin. Not knowing what to do about it I quickly set up our previous 72 gal as a Qt tank and moved all 10 fish out of the 150 display. 3 are tangs, the yellow looks fine as does the sohal who is still scratching around a bit.
The LFS saw the picture yesterday and thought it might be black ich, advising me to do a hyposalinity treatment which I have began. The sailfin has had one 5 minute fresh water dip so far but looks about the same, eating and swimming around with the others. Any help with this mystery would be greatly appreciated.