Hi Beth,
I have a 300 gal saltwater pond in my backyard, with a clear roof to keep out rainwater. Actually it is high brackish - 1.014. I keep clams, oysters, mollies & shrimp in it. This is the fresh food source of some very spoiled fish in my house. The circulation in the pond is pretty strong & the plastic plants I have in it for shelter & spawning were being tossed about. I wonder is plastic coating the weights would be an answer? Stainless steel nuts or bolts would probably work, but I bet they would eventually rust.
Thanks,
Steve