Yes and no. It would look decorative until the algae comes…and that stuff isn’t as easy to remove as in fresh. Also, sand is necessary for a lot of the microorganisms and fish that are vital to marine ecosystems. But yes, shells do look good in there. I scattered a lot on top of my sand, it was good for a while until my pistol shrimp stole them lol.small seashells as a substrate would look cool in a saltwater coral aquarium