What is the preservative they routinely spray on fish displayed at the seafood counter?

I was chatting with the fish guy at the big supermarket I frequent & was shocked when he told me that they routinely spray the displayed fish on ice with some kind of preservative to keep them looking fresh and bright. Frozen fish doesn’t get or need any such treatment though – thank goodness. And there I was, thinking that “fresh” truly meant fresh! I forgot to ask what the preservative was though – does anyone know?