There are several ways to prevent turtles from getting barnacles. First, ensure your turtle pool or tank has a filter system. This prevents debris from entering the tank, which could lead to barnacles growing on the shell. Secondly, keep an eye out for any signs of algae growth on the shell. This could mean there is too much salt in the water, so ensure you change the water every week. Finally, make sure your turtle is fed properly. Make sure they get enough protein, calcium, vitamins, and minerals. All of these things are essential for the general health of a turtle.