An apparatus and method for mixing gas and liquid comprising a submersible or a self contained, self powered floating pressurized dome aeration apparatus housing multi-shaft intermeshed plurality of mixing discs with a remote umbilical power and control unit and process for adding dissolved gas (oxygen) into fluid (water), wherein intermeshed rotating sets of discs operate on parallel shafts driven by variable speed motors or drives, and strakes are radially mounted on the discs to carry liquid up into a mixing area and to carry air and liquid down into a mixing area resulting in a shear force that drives air into the oxygen depleted liquid.