In the case of milk bottles, the absorbed milk may leak back into the water from the plastic and sour the water. Harmful bacteria can grow in such an environment. Therefore, water should not be stored in milk containers. Plastic containers that have not been used before are usually better than used containers. These should be food grade plastics that are made for water storage.
Good containers are readily available in a variety of different shapes and sizes.
One thing to remember is that water is heavy and anything with more than twenty gallons in it would be very difficult to move. One of the best water containers is the metallized plastic bag in a box system. The water is kept from light, keeping algae and bacteria growth to a minimum. If kept in a cool place, the water can be stored for long periods of time. Even with these containers, the water should be checked at regular intervals to make sure that it is potable.