Thank you for your question. Here's a note from our Aquatics Team: The sump level should be between 5 and 10 inches deep. The important thing however is water depth stability. If a person has the skimmer adjusted for a sump water depth of 10 inches and the water is allowed to evaporate down, the skimmer will not skim as well the lower the water depth becomes. Once it hits between 4 and 5 inches I have found that the water in the skimmer drops all the way down to just a couple inches above the sump’s water depth. Once you top off the system the skimmer’s internal water level jumps back up again.