Hope you took him back.
I had a Yellow Tang die on me when in quarantine some months back. It appeared to be going fine (eating well) then I found it stone cold dead.
I wonder where the shops are getting small Yellow Tangs from if they can't collect them from the wild? Have they found a new source? I wondered if they might be using cyanide to collect them? I recently read an article on the percentage of fish imported into America that show traces of cyanide: So don't think it doesn't happen....
They're getting them from outside the state limit which is 3 miles which has pissed off that dickhead Snorkel Bob. Their little ban did very little to stop the collection from Hawaii. That report you read was disproven by several sources including CORAL magazine. They are very close to an instant test for cyanide though which will be nice because we'll have ammo to use against bad suppliers. Now you have to wait for the fish to die, take the liver out and test that.
I've never seen anything from Hawaii caught with cyanide, Indonesia and The Philippines are where that is still used. Some things to avoid out of those countries are Firefish and Royal tangs, you have an 80% chance of getting ones that were juiced with cyanide. I can look at them now and say with certainty which ones were caught with cyanide. It's a damn shame. Some things to look for are reddened gill plates and very vivid colours, they're about to check out.
Safe places to get fish include Australia, Vietnam, Sri Lanka, RVS as your Philippines supplier, Africa, Vanuatu, Tonga, Fiji, Samoa and most places in the Caribbean. Indo is so bad that I won't bring in fish from there. I do have one supplier I use in Lombok for select Indo fish and inverts, shrimp gobies and their shrimp.