Although part of filtration, a skimmer is a separate unit that deserves is own mention and will be new to anyone who has not kept marines before. Whilst it is possible to run some fish-only marine tanks without skimmers, it is not recommended and a skimmer should be considered an essential part of any marine aquarium.
What does a skimmer do?
The job of a skimmer is to remove dissolved waste products before they break down into toxins in order to keep a better quality of water over a long period and prevent build-ups of dissolved organic wastes. Skimmers work by creating a highly agitated flow of water against a stream of fine bubbles – dissolved wastes will attach to the surface of the bubbles and when they rise a foam is formed at the surface which then overflows into a collection area or 'cup' where it can be periodically removed. To work effectively the bubbles must be kept in contact with the water for as long as possible to allow enough time for wastes to attach. Different skimmers have various methods of doing this and a more efficient skimmer will treat a larger volume of water.
Types of skimmer
The most basic type of skimmer is an air-driven unit, which can be powered by an air-pump. These are fairly basic systems and are really only suited to smaller tanks or temporary housing such as quarantine or treatment tanks. A powered skimmer uses a water pump to push water through the skimmer as well as draw water in through a venturi (the water flow draws air down a tube) With this system it is possible to use the flow of water against the bubble stream, pushing the bubbles down and increasing contact time. More powerful skimmers use an impellor with many small blades, often called a pinwheel or needle impellor, to 'chop' up the bubbles and create a mix of water and fine bubbles which significantly increases surface area and efficiency.
Internal and external units
For most tanks, the quality of an internal or external skimmer is roughly the same and choosing between the two will depend on which fits best with your style of tank. The biggest problem with skimmers is that the collection cup at the top of the skimmer needs to be above the water line, and since it is usually several inches high, it is often too large to fit inside the aquariums hood. An internal skimmer will look tidier because it can be hidden by rockwork, but you may need a specially modified tank or choose carefully when purchasing to ensure the aquarium can accommodate an internal skimmer of sufficient size. External skimmers normally 'hang' on the back of the aquarium and pump water out and through the skimmer, which then returns via an overflow into the main tank. The collection cup on these skimmers is usually visible a few inches above the hood once installed. As before, you will need to make sure that the skimmer will fit your chosen aquarium style and you may need to modify or cut away sections of the hood to make it fit properly. If you are setting up a larger tank with a sump, most of these problems are eliminated since the skimmer can sit in the cabinet underneath the tank along with the sump.