There is no real need for an airstone. I never use them.
The idea behind an airstone is that the air that bubbles up creates waves at the surface of the water which equals more surface area for the water to gain oxygen from the air. There is almost no oxygen transferred from the air bubbles to the water.
This is what I've always been told and if I'm wrong I'm sure someone will correct me, hope this helps
I use a bubble wand to break up the nasty film that builds up on the surface of the water.It kind of looks like oil. I think its from feeding beefheart and other meats. Its not thick or anything,but I question how much oxygen transfer can go on between the surface of the water and the air with the oil slick action going on.Just my 2 cents,what I do, and what works for me.