An EXCHANGE TANK is the term used when you give us your used tank in exchange for a Cynch tank containing a net weight of 15lbs of propane. The price for an EXCHANGE TANK is much lower, since we take your used tank with us. A SPARE TANK is just that - a spare, ready to use tank that you purchase without exchanging a used tank. Since you are buying both the propane and the steel cylinder, the price is higher than an exchange purchase.