I never knew what a water tank was.  Only that I knew that you collected rain water over time.  I figured that would take forever and not really be worth my time.  However, after using it for one summer at my grandfathers house I realized the benefits of these devices and started using them at my house.  I soon learned that they collected rain from your gutter systems.  Which makes so much more sense than just sitting something out in the yard.  The rain water tank that i have holds 50 gallons and can last me through the hottest months of the year when we have to conserve water.