The "plug" in the upper photos is one of those inserts you jam into a Philips head screw to hide it. It makes an excellent plug for the unused air hose connector. Left, lower shows two black rubber plugs that came from the AeroGarden Harvests when reservoirs were installed. Lower right shows one of those plugs in use. A short nail with a wide head would do.
The decision not to aerate is not irrevocable. I reserve the right to change my mind on a whim. While I firmly believe that bubbling air through its roots saved the mini-jalapeño, which is the one shown lower right, I also believe all that extra oxygen was the source of the algae blooms and mold in some of the jars and Harvests.
That said, the air hose hole makes an excellent port through which to add water and nutrients to a Mason jar without the need to unscrew the lid or disturb the plant.. It just needs to be plugged to minimize air that causes an algae bloom. A "green slime" incident. I'm very tired of those.