210 build..a problem arises..advice needed

I got a used 220 im about to begin on ( had to do some work on stand) and it should be starting setup by tommorrow..

..the problem that has arisen ..about 10 to 12 days ago late one afternoon I noticed my yellow tang kind of hovering behind a rock just above the sand..I watched him a lot and he was always strange anyhow..didnt notice anything till then he was normal and eating and everything the day before and I dont recall anything strange that morning..anyhow over several hours it was obvious he was not well..just kind of low energy and eventually listing a little..he was till vibrantly colored and I could not spot one thing on his body or gills..his breathing may have been a bit faster but nothing obvious..since I saw no signs of a particular disease ( im a newbie) and i didnt have meds that evening I thought it best to leave him in there..as stress of pulling him out with nothing to treat on hand..and no hosp set up..he was gone and on the powerhead the next morning..( poor guy) I have no idea what it was..I have watched my other guys 2 clowns an coral beauty and a diamond goby..and seen nothing obvious so far.

.I was planning to use that rock to help cycle my new tank-..guess that is out..

i have a cleaner shrimp a pepp shrimp a couple snails and like 2 crabs..and one plate coral..now im wondering what i should try to do ..as i sure dont want to transfer ich or something to the new tank..advice from experienced folks much appreciated..I have ideas..but Id hate to treat with copper and lose a fish when i didnt even know what killed the tang..would love some advice....