In my DWC I fill the revervoir to the bottom of the basket and zip-tie the airstone to the bottom of the basket, as well. That assures that the developing roots get plenty of oxygen as well as assures the bubbles splash enough water to keep the jiffy plug moist. Once the roots are sticking out the bottom of the basket, as shown here, I cut the airstone loose and allow it to sink to the bottom of the reservoir.