I have that system. My setup is a pair of 1 reservoir/4 waterfarms. I use ~8 gallons of water in each setup. For each farm that puts the water level at ~4.5 inches and leaves about ~2.5 inches of air.

I have a pump in the res that sends water to each of the 4 farms thereby circulating the water. I added another of the same type of air pump and run 2 lines with airstones going to each res.