I have a couple more thoughts on this:

as a business owner - what's one sailfin tang in the grand scheme of things $$$ wise? Nothing. Somebody running around town telling all their friends they got a sick fish from you that died before they even got it in the tank ... priceless in a bad way. It comes down to 'Do you want to be right or do you want to be happy?' You are probably right in that sitting for hours in too small of a water volume caused its death. But what will give the happier outcome for your business? Probably taking care of it regardless. And then creating a store policy you can live with.

as a customer - I am with NateDog there. I just lost a pretty expensive fish. Very sad. It never started eating. I even had to tell the phone support lady to stop calling me because talking about it just made me feel worse. The store I bought it from has some kind of warranty policy I think but I have never planned on going that route. When you buy livestock, stuff can happen. I don't hold the store responsible for it. Not sure what happened.
BUT: I think I would totally be offended if I did try and claim it and was interrogated or lectured as to why it was me who killed the fish. True or not... there's just a way to put things without making the customer feel like an idiot (even if we sometimes ARE idiots)

Like in this case I might phrase it like: "Tangs are adapted to live in highly oxygenated waters. The water volume we supply in the bags is sufficient to carry the fish home but it needs to be acclimated immediately as it is not sufficient to sustain the fish for prolonged periods or time."
And then I'd offer up another sailfin tang for this case.
I am lucky in that my business is online so when I get a customer complaint I have the luxury of walking away for a little bit, cool down and think about the most productive response.
I bet it's hard when you are face to face with somebody and need to come up with a response right then (and not show in your face that you really think: you idiot lol) So that brings the importance of that store policy back up. Get one. That way you have a starting point on how to respond when it happens again (and I am sure it will)