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 7 days.
Is it normal for medication to take this long? Any ideas how to get rid of it?

[respite]

Freshwater dip should be the solution.

http://www.wetwebmedia.com/paravortexfaqs.htm