In a static system, with no forced circulation, the water at the surface will quickly reach saturation conditions for oxygen dissolving in water, as determined by atmospheric oxygen and dissolved oxygen partial pressures at the given temperatures and pressures. On the other hand the water will tend to stratify in the tank and oxygen levels far from the surface, even a cm away from the surface, will not reach that level, assuming something is present to remove oxygen like fish or corrosion processes or even plant respiration. If the water is prevented from reaching a stagnant condition, then all water will eventually come into contact with the surface and thus will acquire saturation conditions when it does so. As long as that circulation is significant, the only benefit of oxygen being directly absorbed from a bubble's surface would be that it can happen deep in the water column, not just when the water returns to the surface. Net oxygen absorption from that bubble will also stop when the water immediately surrounding the bubble reaches saturation. Any fish respiration should be small compared to the rate of circulation of the water, which is often given as a minimum of 4 times the volume, or an average of every 15 minutes having a full exchange. I find that the only benefits of bubbles would be the slight increase in oxygenation in partially depleted water deep in the water column and of course the increased rate of circulation itself caused by the buoyancy of each bubble.
