Surface movement helps carbon dioxide (CO2) come out of the water and allows oxygen (O2) into the water. The more surface turbulence, the better this happens. It happens even when the water is perfectly still too but it doesn't work well when the water is calm and flat.
An airstone bubbling away helps to circulate the water in the tank as well as produce lots of little bubbles that break on the surface, and this creates the most gas exchange.
If you have lots of fish in the tank and not much surface turbulence, the fish could suffocate or have problems with too much CO2 in the water.
As a general rule, having lots of surface turbulence is better than no turbulence.
Ok so I got an air pump and stone and I have had it going all day. I figured I should turn it off at night to let fish rest. It takes up roughly half the back of the tank. I'm thinking it will irratate the fish at night. What do yous think?