Chow_13answered grow question 2y ago Water level depends on what your using as your growing medium. My grows are 100% clay pebbles ( NO rockwool,rapid rooter to germinate. I place the rooted seed 1" down and cover with clay pebbles. Top water once a day for 4 days) Water level I have even with the bottom of the 6" net pot and have no issues. But if you germinate in something that holds water and that is placed in the clay pebbles then I would go with 1-2" below the net pot to avoid drowning as the rooting material can get over saturated.
Air pump is needed from day 1( to splash water onto the net pot and keep the water from going stagnant. If you have no water movement/ no oxygen in the water it will promote root rot. Technically you don't need an airstone and can get away with top watering daily until the tap root drops into the water. But adding air you get way more growth, ward off root rot and can grow a bigger plant. Lookup "Kratky Method" to see a no air stone setup.
Happy Growing