Hey all, here's some better pictures!
That's marine ich, Cryptocaryon. There are other syndromes that cause salt-sized white spots on tangs, but the presence of spots on the clear portions of the fins is pretty definitive for ich.
Do ANY other fish in the tank show spots? Powder blues are notorious for showing ich in aquariums that generally don't have a big infection going on. If you don't see any other spots, you might get away with ich management in this case:
1) Use a strong UV sterilizer
2) Perform frequent water changes
3) Siphoning the sand early every morning
4) Good mechanical filtration to remove theronts
5) Low dose peroxide additions
6) provide a good diet - but in this case, the tang is pretty hefty, so the diet isn't the issue.
Ich management only works if you catch the infection early enough and do it right. Once the number of trophonts on the fish reach a certain point (perhaps 30 or so spots on any one fish - and yours may be past that point) then "propagule pressure" comes into play, where the effects of the trophonts themselves stresses the fish and the ich management techniques start to fail.
If management fails, then you'll need to move to a proactive treatment phase where all of the fish are treated in isolation from invertebrates with either copper or hyposalinity.
Jay
That… not reefmedic…
