Re-filling Soda Stream CO2 tanks must be done commercially. When a tank is empty, it can be exchanged for a cost at dealers that market Soda Streams, such as Bed Bath & Beyond, Kohl's and Costco. A complete list of marketing outlets can be found on the Soda Stream website.
The CO2 tanks are referred to as carbonators and are used to inject the carbonation into water, which can then be flavored. They are sold in two sizes: 60 liter and 130 liter. The liter reference is to the optimum amount of carbonated beverage the tank can produce before needing to be replaced.