parasite problem

[kevin254]

I bought a yellow tang and after two weeks of being in quarantine, black ich showed up. I lowered the salinity and after a few days all the spots were gone. One week later 2 black spots reappeared. These two spots have been on the fish for about 3 weeks now. A few other spots have come and gone but these two have been constant. I've keep the salinity low (around 1.008-1.010) and used Rid-Ich for about 5 days, then Sulfathiazole for about 6 days, and now using ParaGuard for about 6 days.
Is it normal for it to take this long?

[kevin2000]

Black ich is not the same as std marine ich.

A series of FW or Formalin dips maybe in order.

http://atj.net.au/marineaquaria/blackich.html