For storing water long term add 4 drops per gallon of water being stored. Use the enclosed 10 mL (1/3 ounce vial) for measuring. One vial treats 55 gallons.  It is recommended that you change out your stored water once every 5 years. Berkey Biofilm Drops™ remain active in water being stored for years and Berkey Biofilm Test Drops™ can be used for testing residual levels of the Berkey Biofilm Drops™. Berkey Biofilm Drops™ have an indefinite shelf-life as long as you don’t freeze them