An acrylic epoxy. I have a jelly fish I bought from a vendor that is completely encased in an acrylic epoxy (like glass paper weights with flowers in them). I keep it in my glow tank and it has not affected the water parameters or the fish. I've had it in the tank for over a year. I don't know much about how to use the acrylic epoxy but I am sure you could find something in a google search.