By dropping the water level anything bellow the net pot you are robbing yourself of space for your feeding roots to grow. As mentioned above, only big thick roots for support and nutrient transport grow in this space, not roots that feed, which is what you're after.
Having an air gap is a misconception of DWC, run without one for best results, if you don't believe me, google Heath Robinson, the guy who invented DWC as we know (by accident)
The need of an airpump is another well known fallacy.
Notice how all the roots bellow the water are nice fine feeding roots? Then the ones that came out the sides of the netpot are thick and gnarley, because they're above water level, I'd be better off with a "net pot" with holes on the bottom only, which is what I'll do next. Dropping your water level anything bellow the bottom of the netpot will rob you of root space. Your plant turning it's roots into big thick cord roots to survive, and wasting energy doing so. More root space, less cord roots, and less energy stolen for a survival mechanism means bigger fatter buds for you at the end.