All AGs come with a Water Pump. What it does is circulate the water inside the reservoir, and also feeds it up top to flow over the sponge-inserts. This keeps the inside balanced, and keeps the top of the plant slightly moist
Air pumps, via tubing into Air Stones, create oxygen in the water. Oxygen = plant food. At standard places, Pump + Tubing + Airstones should run you around $10-20. They're not necessary, but help a lot!
However, as I learned the hard way, TURN OFF THE AG WATER PUMP once there is plenty of roots in the water. Otherwise, the pump might suck in the roots, which will lead to root-rot, poor performance, and weak plants