All through the '80's I had 2) fifty gallon tanks, 1) twenty gallon, 1) ten gallon and 1) five gallon (all fresh water), aquariums going. I took a small (about 4" long) 3-masted sailing ship model, (was molded in white plastic) built it. Cut a hole low in the hull. And broke the masts and cross bars in a couple of places, and glued them on the deck as though it had been sunk for years. I placed a few pennies inside the hull to hold it down. And for several years had no problems at all with it bothering the fish. Course I had NOT painted mine (after all if it had been sunk for years the paint would be gone anyway). After a couple of weeks the algae turned it a nice green, and it looked real good. I don't know if the paint would cause any problems or not, (I wouldn't take the chance myself) but I know the glue and plastic didn't cause any harm. I would wait a couple of weeks before I put it in the tank just to make sure the glue is fully cured. Kasy