Clown tangs come from very shallow, very rocky, and extremely turbulent areas of the shoreline. I believe sea water only has a certain 'natural' oxygen saturation level. But the by-product of have very high flow rates in your tank, this causes what seems like 'highly oxygenated water.'
Their location causes two main problems for the aquarist. First, because of the rough water, they are extremely active and need monster tanks with a monster turnover rate. Second their location makes it difficult and dangerous to collect them. Thus, IMO, is the main reason some of these guys fail in captivity, cyanide collection. This also IMO, explains how some doing very well, (obviously not EVERYone is collection though cyanide) and some dying quickly or linger on for a bit, then die.