Surface tension of the aquariums water.

With in the first 5 mil of surface water,even if the water is just standing still in a bucket open to normal air, the salt water is oxygenated in standard salt water of roughly -25 salinity-24 degrees temp-and near to no pollutants,eg,ammonia,nitrites,nitrates,protein and so on, if conditions are normal it will happen(no matter what).

The tension in salt water means that gases are trapped in your water unless in the simplest ways to release these gases are not in action and that means your water needs to be moved in a violent manner while making contact with the air.

The ocean uses waves, very strong open ocean currents or tidal currents.

You need to mimic at least one of these in your own ways.

The thinner you can make your water when it makes contact with the unpolluted air and achieve this in the most violent of manners possible, than the water will pick up oxygen and than the nitrogen, ammonia and other unwanted gases will be forced out of the water into the air.

Protein skimmers can not be relied upon to do this in a very affective manner,the bubbles on avergae are there as long as possible to become coated and to not explode during suspension.