My system is around a month old. I had the start up quick because I used fully cured live rock and hence started adding bioload up. I got a brittle, 2 o. clowns, 1 firefish and 2 turbos along with some frags. The other fishes are doing fine.
I don't thing its a issue of oxygen since I have enough aeration (2 aquon 20 HOBs, 1 rio nano skimmer and a 110gph ph).
I guess this is the common issue of getting a wild caught fish
I shouldn't have got him when I heard that he was wild