If you have bleach stored to purify water in your long term supplies, did you know that bottles of bleach lose their effectiveness by 20% each year they are stored? Make your own from concentrate. Takes up less space, there when you need it at your pace. Get the instructions on how to use it from Living Prepared. Do not purchase Pool Shock that includes any additives such as algaecide’s, anti-fungal’s or clarifiers. All you want is “Calcium Hypochlorite” and the rest being “Inert Ingredients”.