although it is true that the majority of gas is exchanged at the surface, there is a gas exchange at the bubbles. the finer the bubbles the better. it would be interesting to compare the agitation created by a 20watt air pump verses a 20watt water pump and the resulting DO. since water falling back in from one tank to another would not produce the greatest circulation, the water pump would have to be located at the bottom of the tank pumping straight from the pump outlet and not a hose.

using a power head at the surface, the water stream is weakened by hitting the opposite side of the tank and the circulation of water in the tank is incomplete. the most effective direction is from the floor of the tank at one of the long ends of the container aimed straight up. it will create a complete circulation of the water. you can check this out with dyes added to the water.