A common practice for rain harvesters is to use their units in a cell-like fashion where several units connect to form one large storage unit. A common practice for rain harvesters is to use their units in a cell-like fashion where several units connect to form one large storage unit. The benefit of this method is that one can effectively increase the amount of water harvested from a single point without needing to change the gutters or position of the units. Most of the tanks on our list can connect in series to overflow into each other. However, if you’re looking to make a proper series, the best models have specific overflow pipes that easily funnel the water from one barrel to the next.