The air gap is the space between the nutrient solution and the bottom of the net cup. It forms as the plant uses water, and it is what makes the Kratky method passive.
How it forms
Plants take up more water than nutrients, so the level drops. The roots that are now in the air can take in oxygen, while the lower roots keep drinking from the solution.
What to watch
Check the solution level every few days. A small gap is normal. If the solution drops so far that the roots dry out, or if the level never falls, check the set-up and the plant’s growth.
When to top up
Top up with plain water if the level falls too far. Avoid topping up without checking the pH, because the solution can drift.