Rain tanks are like rain barrels, only upgraded: bigger, better, and your harvested rainwater lasts longer. While most rain barrels hold about 50 gallons of water, rain tanks can hold 1,000 gallons or more, capturing water from several storms and storing it for use over the course of many drier months. WMG will show you how to use rain tanks as an active form of rainwater harvesting to nourish your vegetable and herb gardens throughout the year.