It's like aerating a fish tank. If you don't have a compressor adding bubbles, the fish hang out at the surface, gasping for the small amount of air absorbed at the surface.
On systems that have a compressor, there is still an method of blending air and water for proper aeration. Just having air above water in the precipitation tank is not adequate. The Iron Curtain uses a special spray diffuser inside the tank to aerate the water.