Generally, with good water movement you wont need an airstone, but it isnt to say they dont come in handy. As Sf05 said, they are useful for removing any surface scum on the water's surface from food. I also used one in a fishes shipping bag when it was struggling after being shipped a long period. Good for emergencies id say. If you can afford one, id say buy the pump, airline and stone for safety, but I wouldnt include the airstone in my actual setup 24/7