Sometimes, white spots on a fish  isn't marine ich (cryptocaryon) and so don't jump to that conclusion or you may end up treating the wrong way and stressing your fish and yourself out. The Paracanthus tang is the poster child for an opportunistic external bacterial infection (vibrio sp.) which I have diagnosed a few times with a microscope and staining. I have had numerous cases of large tangs with white flecks or little jelly pinpoints which looks almost exactly like the classical cryptocaryon presentation but is bacterial in nature. An antibiotic food is a good option. One that absorbs in the gut (so kanamycin and neomycin are not going to help) and one that treats gram negative bacteria (so metronidazole wont work either). If you are going to skip diagnostics with a veterinarian involving a culture and sensitivity of the bacteria isolated, the ideal drug is a flouroquinilone like enrofloxacin or ciprofloxacin but a tetracyline might work too (minocyclin) and you can message me if you are interested in this route. Ideally you catch the fish, do a skin scrape and look at that under a microscope scope (with stains if you don't see any protozoans), and give the fish a bath . An hour in kanamycin should work well it was in fact bacterial. Keeping the fish in a quarantine tank or even a bucket for 5 days after catching it and either medicating the holding water or doing further dips (every other day with the kanamycin) is the best way I have found to beat these bacteria, see the section on the website on medicated dips or baths or email me for a quick sheet. If you do end up finding cryptocaryon, first, I'm sorry as this is generally more dangerous and is harder to treat but there are accurate resources and links on healthyaquatics.com for dealing with this as well.