a thin coating would only be able to reflect a very narrow band of frequencies. foam is used because it has all different shapes embedded into it, which can absorb many different frequencies.
so there could be such a product, but it's not as effective as the user might think.
I didn't have fish when i was in a band...but from what I remember,
live rock music could probably liquify their brains! Fish generally ignore all kinds of noises, but such LOUD noises (like really loud!) would definitely stress them out.
I always wore ear plugs...fish don't have that option.