The speed of the pump will change the water flow into the body of the skimmer but will also change the amount of air by ratio. This happens due to a higher negative pressure being formed at the venturi stub as the flow increases, thus pulling in more air. 

 

At each adjustment of the speed setting the ratio of air to water will remain more or less constant. The higher the pump speed setting the more water and air will be introduced into the body of the skimmer.

 

Once you have set the speed of the pump you then need to set the water level inside the body/neck for your desired skim, either dry or wet depending on the bubble break level. Each time you change the speed of the pump you will need to adjust the water level back to the previous height in the neck if you wish the dry/wet skim to remain the same as the previous pump setting. 

 

The reason for the different pump speeds relates to the nutrient loading of the tank, when using the skimmer on a tank with lower stocking one would set a lower speed, the lower speed would equate to a longer dwell time within the body and increase the efficacy of low nutrient removal. In a higher nutrient system the dwell time wouldn’t be so important as the higher nutrients would be more available to the skimmer, making it easier to skim out. In this case one would favour bulk through put over dwell time.

 

In each situation the water level would be a final adjusment for your prefered wet or dry skim however, as already mentioned, regardless of the pump speed.