Protein skimmers, on the rare occasion, can start behaving erratically and produce so much foam that the collection cup overfills in a very short amount of time. This action can be brought on by adding a new media to a filter, swapping out an old filter sock for a new one, or when a tank resident dies. Regardless of the reason, if the collection cup already has a good amount of skimmate already in it, the tank can suddenly be inundated with the nasty smelling gunk. To make matters even worse, the protein skimmer will act even crazier once all of this happens, often overflowing again. In most cases, the skimmate will usually not cause any short term damage to the inhabitants of the aquarium, but it is highly recommended to perform a water change as soon as possible.