Hi gmiller, it's probably one of two kinds of parasites. Cryptocaryons (classic ich) or Amyloodinium (marine velvet). What you're describing sounds like a bacterial bloom, normally associated with newer systems, but it can occur in tanks of just about any age. What you'll actually need to do is NOT do any water changes, but address your nutrient export issues.
You'll also need to get the tang into a quarantine system and begin treating it by using hyposalinity (1.010 or so). This will kill the parasite by the difference in osmotic pressure. Use our search engine for "ich" and/or "hyposalinity", as well as searching Advanced Aquarist for the like. In the meantime, keep the display fishless (completely) for 6-8 weeks (8 are better). The parasite is an obligate parasite, this means that it MUST have fish to survive in the system. No fish = no more parasite (assuming it's Cryptocaryons--which looks much as though your fish has been salted).
Any watertight container can be used for quarantine, a quick and dirty setup would be a Rubbermaid container with a small heater and a sponge filter or hang-on power filter. Plan on copious, frequent water changes to maintain acceptable water parameters.
Best of luck!