Air Stone / Air Curtain
I've always had an air stone in my 180 litre goldfish tank and it helps to keep the surface of the water moving, especially at the other end of the tank, opposite side to filter outlet.
I've just replaced my old air curtain with a new one, which produces a lot of very fine bubbles. Is there such thing as too many bubbles? The 3 fantails don't seem bother by it and I'm sure they like swimming through it, but there are a lot of fine bubbles floating around in the water.
I have a normal air stone that I did think about putting in at the opposite end of the filter outlet, just to keep the far end of the tanks water surface moving , so as to be in a more concentrated area, rather than an air curtain the length of the tank.