A method and apparatus for dissolving a gas into a fluid which may contain at least one other dissolved gas. The apparatus includes an inlet tube for receipt of the fluid from a container containing the fluid. A gas inlet is operably connected to the inlet tube for the introduction of gas into the fluid. The mixture of gas and fluid is introduced into a gas transfer device via the inlet tube. The gas transfer device is positioned below the surface of the fluid in the container so that the gas transfer device is hydrostatically pressurized in order to increase the rate and concentration at which the gas is dissolved into the fluid. The gas and fluid mixture is allowed to flow downward through the gas transfer device such that bubbles of gas are dissolved in the fluid. The fluid having the gas dissolved therein enters an outlet means positioned within the gas transfer device. The fluid having the gas dissolved therein flows upward through the outlet means and is released from the gas transfer device while the bubbles of gas which are not dissolved in the fluid are retained in the gas transfer device.