Replacement Air Filters for your VISO II, OPTIMA II, or SOLO II (Includes 5 filters). If your L&B altimeter/audible spends time in the pond, it is a very good idea to replace the Filter. Our audibles/altimeters are water-resistant but it is advised to replace the filter after a dunk in anything but clean water (including pond water, salt water and any other pond that has debris of any kind). Replacing the filter is easy but requires a special tool. If you do not have the filter tool already you can purchase a filter & filter tool pack from this webstore. Attempting to remove/replace the filter without the tool is NOT advisable and could damage your audible/altimeter.