I think he meant how deep the water level in the sump should be.
Here is the info I found in a thread on --...
This is an e-mail from Octopus. Re a NW-200
Thank you for your enquiry, there is two important water levels to consider.
Water level in sump: The water level in the sump should be about 3 inches below the start of the neck line on the skimmer body, this is important so that there is enough water pressure for the pump to draw. It is important that the water level in the sump does not change dramatically or else it could cause back pressure and less bubbles would be created in the skimmer chamber.
Water level inside skimmer chamber: The water level inside the chamber should be around the start of the skimmer neck line. The way to achieve this is to adjust the water outlet pipe, by raising it up you would increase the water level inside the chamber and by lowering it you would reduce the water level.