Protein skimmers come in many different forms, but they all consist of a chamber that water is pumped through that is heavily aerated, organic pollutants stick to the surface of these bubbles. The pollutants form foam that then climbs up a neck and into the collection cup where it collapses into a dark coloured liquid. While it is possible to maintain a marine aquarium without a skimmer. When you see the smelly dark liquid they collect, you will most likely agree with us that is an important bit of equipment for marine aquariums.