Fast respiration is only a sign of fish stress - and by that symptom alone it is not possible to determine any thing persee. Also, shortly after death, parasitic infections usually depart the host rather quickly and return to the substrate/ind another host. Without blood flow they cannot feed properly and depending on the stage of development they were at when the host died, they either find another to complete their feeding cycle, or cyst and drop to the substrate to multiply...
The cause could have been infection of some type, viral or bacterial, but now it will be dificult to identify anything.
Big white splotches you mention are pictured and around the gills and pectoral fins? sounds like just areas of mass color loss, and may not necessarily mean any specific cause.
Usually the best identification happens just after incept of ailment and before the fish is so sick any treatments would not be any good, as he's too weak to survive treatments.
Try to identify any ailments on the other fish before they go too far down. Look at the FAQ section here to help identify anything. Tangs are most susceptible to skin/gill parasitic infections that leads to death rather quickly with Ich and with marine velvet the timeframe is measured in hours!
Was he eating much in the last 2 weeks? What diet were you providing him?? any new tank additions (coral/invert/fish/livefood) that came from ANY other tank in the last 2 months??? Was he scratching on rocks/substrate prior to death????
