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If you have sufficient surface agitation then no airstone is needed. Usually it's only needed if your filter output is so far underwater the surface barely moves
A power filter rated for a 75 G in a 55 is plenty of filtration and gas exchange. The airstone makes very small bubbles and does little to really move the water's surface. If you want to up the surface movement, use the air pump and attach a length of plastic tubing to it. Run the tube without the airstone down the side of the tank to the bottom and attach it with a small suction cup. The much larger bubbles will definitely move the surface water.