They will take over the tank eventually. Physical removal, or use a syringe filled with a strong base and inject them with a drop, it will die. Take dry baking soda, heat it up in an old pan till it looks like it boils, it will have a pH of 14 now instead of 8. Use a little with water and inject the pest with it. Save the rest of the powder till next time. Don't breath any of it, if you touch it, wash your hands. Once you inject it, it will precipitate calcium out of the water. Don't let the fish nip at it, it will shock kill it. Wave it around till the drop neutralizes in the water.