Tank lights have a UV component. Pucks are vulcanized rubber, which UV breaks down.
The major problem of people wanting to put decor into a tank is that a healthy oceanic environment quickly coats everything in pink or brown. Mostly pink, given a smooth surface. So it will rapidly become indistinguishable from rocks, except by its shape.
My advice would be to find a pretty little enameled metal team medallion, the sort you might use for a car, say, and velcro it to the bottom front glass.