The idea is that in some reef systems you CAN overskim. Usually these are implemented in "berlin" systems. In Berlin systems, reefkeepers want to make the system as unobtrusive as possible. They prefer natural filtration and as little skimming as possible. Berliners have claimed to keep types of coral that are rarely successful in traditional "skim it all you can" systems. Overskimming CAN remove nutrients and microorganisms from the water and it can harm some corals theoretically... but IMO, overskimming is a non-issue.