Air exchanges in the average tank primarily through surface movement, not bubbles, so it would be more effective to make sure that the tank is on a level surface and then use an airstone *away* from the opening, so it takes the air longer to leave the water, which means that each bubble temporarily increases the surface area of the tank. If the tank is leaning backwards, then air will collect at the front, so it's probably a good idea to check with a spirit level.