Whether you have a single rain barrel or a 3,000 gallon poly tank, you will most likely reach a time when the supply of rainwater exceeds your storage. In this case, you may be compelled to purchase an additional storage container. There are a couple different methods to connecting multiple tanks so that you can increase your storage capacity. The image below shows both methods of connecting multiple tanks.
The first method is connecting the tanks at the top. This involves using the overflow from one tank as the connection to another tank. The first tank in the succession fills and then its overflow fills the second tank and then third, etc. The last tank must have an overflow.
The second method is connecting the tanks at the bottom. This allows the tanks to all fill and be drained at the same level. If using this method, you must be sure to include isolation valves so that if one tank springs a leak, it will not drain all of the tanks in the succession.