The Ista CO2 Aluminum Cylinder (1 Litre) features a Face-side connection, for easy, convenient installation. It is DOT approved, safety and quality assured, and its surface has been treated with an epoxy resin coating, to prevent oxidation. Easily refilled, this cylinder will prove to be a long-lasting addition to your aquariums CO2 setup. Please Note:Ista refillable CO2 cylinders are DOT certified and stamped DOT-3AL. The bottle threading is identical to North American paintball CO2 canisters, however some refill stations may require an additional o-ring for proper refilling.