Well a power head is what nirbhao says but some powerheads are to strong for the fish you keep, they both ripple the surface of the aquarium which creates oxygen for the tank, if you have a good filter it will ripple the water just fine and you will need neither, I use a big airstone only for the summer when it gets extremely hot for as hot water consumes oxygen faster so I have the stone in there I can turn on to get more surface break to oxygenate the tank more, with an airstone you don't have to worry about the fish and the current.