The way to adjust the filter outlet is as Chris says, so that it causes ripples on the surface.
Gas exchange occurs where the water meets the air; that is, at the surface. Oxygen goes in and carbon dioxide goes out (unless you have a lot of live plants which use a lot of the carbon dioxide). If the water was completely still, gasses would have to diffuse through the water to and from the surface and this would limit the amount of fish you could keep. But if the water circulates round the tank it carries the gasses to the surface and down to the bottom of the tank so the gasses move in or out of the water much quicker.
The easiest way to circulate the water is to have the filter outflow moving across the tank just under the surface. This pushes the water that was on the surface to the bottom, and pulls up the bottom water to the top. Air stones, deep sea divers etc work by the bubbles churning up the water and mixing the top and bottom. Very little gas exchange happens from the surface of the bubbles themselves. Air stones etc are mainly aesthetic.
You could test your filter outflow by putting something harmless in the tank and and watch what happens to it - a bit of polystryene, a feather, something that won't add anything nasty to the water and is big enough to catch and remove.