Sorry to hear that, Scottatmiles. Do the other fish seem okay? You mentioned the new clown to replace the one that died, but were there any other additions to the tank within the last month or two?
I doubt it was an oxygenation issue, personally. Your skimmer can provide some gas exchange provided that the stand isn't completely airtight. And IME fish w/ low oxygen tend to swim at the surface with their mouths up at the waterline. I think you would have mentioned that if it happened.
Since I don't know how long the fish were together, I'd venture a guess that the clowns weren't sexed properly. If you get two females, they will fight for territory and will stress each other. One might have been stressed, which led to a weakened immune response, and then infection, which spread to the tang. From the photos, both clowns seem to be of similar size, so it's possible that they were fighting. Again, this is based on not knowing their history. Was the 2 inch clown that died the larger or smaller picasso?
I think I see a maxi mini anemone in the 3rd pic. Did the clown ever go near it? I've seen several stories of clowns attempting to host, then being killed by maxi mini anemones. They're not compatible, so it's a calculated risk to keep them together. And captive-bred fish aren't picky about hosts, so it's not a total shot in the dark to think it might have gotten too close, been stung, died and passed something on to the tang.