There was a hollow ornament that i really wanted to use. fear of the water stagnating made me hold off. i experimented with placing a small air stone inside, and drilling a hole in the top, for the air, and water to escape, and thereby circulating water in it, and increasing my aeration as well. this particular item was too light however, and the air would lift it and it would fall over. i then tested silicone, to see if it was buoyant. it was, so i went back to the drawing board. i mixed large grained sand into a tank safe silicone, and filled the ornament with it. water can no longer pool inside it, so no stagnation, and its heavy enough so that my more playful fish dont knock it over.
when mixing the sand into the silicone, you have to be careful not to mix air into it. and when filling a closed ended item, like a ceramic, i used a wire to make sure the silicone/sand mix got all the way into it, leaving no pockets of air.