If its made from resin or plastic or even fired clay its ok. i have resin rock and a couple of resin caves and coral structures in my qt so the fish can hide and feel safe. no prob. clean in fresh water and scrub with a brush. as long as its inert and does not absorb any thing, your good to go. you might find that your stuff wants to float though. salt water is more boiant then fresh. get the drill out. by the way, if you use clay decorations, you have to be careful about meds., they can absorb very small amounts.