So Sam you're suggesting that essentially 9 out of 10 yellow tangs on Earth are in captivity? And more for clownfish? I disagree. Or do you mean 9 out of 10 yellow tangs in captivity die in captivity? In that case, I still disagree, I would say 10 out of 10 tangs in captivity also die in captivity. And in response to Brad's comment, I agree, anemones in captivity, when they die, generally also die in captivity.
Ok I'm done now.
As far as I know, anemones (cnidarians in general, in fact) never die of inherently internal causes. I bet that there are corals out there that have been around since Columbus was cruising around Florida.
Seriously though, three dawgs, I would stop buying anemones for now until you locate the source of the problem (actually I would have done this after the first one died). Have you bought all these animals from the same store? Is the store reputable? Were they healthy when you bought them? There are a million questions that could be asked here.