Sometimes the pins on a needlwheel protein skimmer impeller break off during normal operation. Snails, rocks, and other debris get sucked into the intake side of the skimmer’s pump and cause some damage. The impeller though will still operate at acceptable levels even if there’s a few missing needles. Just monitor the bubble output of the pump and make sure the skimmer continues to work normally. If need be, you can easily replace the impeller or just mod it by adding mesh.