depends on your bioload and how much gunk it is pulling out. If it is pasty and dark sludgy then you may want to lower it and make the skimmate a bit wetter, if it is watery and filling too fast raise it up. You will find your sweet spot and be able to judge where you want it. I lower mine after I do maintainance so it is skimming heavier and at other times I will raise it so it skims lighter.......