Nothing special ...just a basic 4" cylinder air stone. I am using the HTG Supply Bubble Boy single bucket top. It has the 6" net pot built in to the lid like some others do. The trick with this lid is that the net pot has a channel that comes up from the bottom of the pot. You place your rooting plug on the top of this channel and pretty much once the tap root is out of the plug in is already getting misted on from the bubbles in the bucket and doesn't have to stress while it finds it's way through the hydro stone. I think it made my life easier too as I only top fed for a couple of days at most. Seems I see a number of new hydro guys drowning seedlings by to much top feeding at the start. What I did was I cut the pot mesh away from the area directly under the root plug as well. So basically there is a 1" hole straight into the bucket for the tap root to go through.