In general terms, a protein skimmer is a piece of equipment that uses a mixture of air and water to create a column of tiny rising bubbles that "grab" skum, film, and dissolved organic waste materials (DOC's) by binding those pollutants to their surface. The bubbles take the DOC's to the top of the skimmer, where the bubbles pop in the collection cup. Left without lift, the pollutants are marooned in the collection cup where they accumulate until you empty them. The efficiency of your protein skimmer depends on how hard the bubbles are forced into the chamber (harder makes them smaller) and how long they stay in the chamber (longer contact time means more contaminants are picked up).
Saltwater and reef aquariums are managed to much higher and more exact standards than freshwater aquariums because the fish and invertebrates are more easily stressed by poor water quality. Although nitrate (the end result of biological filtration) is less toxic than ammonia or nitrite to most fish species and invertebrates, corals cannot tolerate even the small amount of nitrate in a well-managed tank. Because the skimmer pulls out debris and detritis before it has a chance to change chemically into toxic ammonia and nitrite then nitrate, the tank stays cleaner longer. Biological filtration is aided. Water changes are minimized. And the sensitive species stay happy.
Whether you should to use a protein skimmer on a reef-only tank is still controversial. While it is a definite benefit in removing phosphates and detritis, it also removes some trace elements needed by some corals and invertebrates as well as some natural food sources such as plankton. If you use a protein skimmer in a reef-only or reef and fish tank, you will want to take the trace elements into consideration as well as turn it off for a few hours each time you introduce phytoplankton.
Protein skimmers also confer several other advantages. By disposing of debris before it pollutes the tank (or turns to nitrate), the tank will have better light penetration and less algae growth. The aquarium will also have higher oxygen concentrations which will help to stabilize the pH level. The filter, by virtue of the fact that less DOC's are being filtered, will operate more efficiently.
The additional benefits of a protein skimmer beyond just skimming sludge make it a very wise investment indeed. Your tank will be more stable, more beautiful, less work, and less expensive in the long run.
It is a well-accepted fact that marine tanks of any type and size need a protein skimmer. The higher the fish load, the larger the skimmer must be. Try to purchase the largest skimmer possible that fits within your budget and allotted space, as the more skimming you can create, the lower your maintenance duties and the healthier your tank will be.
There are several types of protein skimmers. Each has its advantages or disadvantages.
The major drawback to this type of skimmer is that the air stone or diffuser is likely to get clogged and ruined with salt accumulation and detritis. When that happens, the pump is put under tremendous back-pressure (often damaging or destroying the pump) and air bubble production is severely limited. No bubble, no skimming. Limited to tanks under 20-30 gallons.
Venturi protein skimmers are usually smaller and quieter. They use a venturi valve to create a high-pressure jet of water that draws air into it as it passes through. The higher the pressure of the incoming water, the more air is drawn in. More air means more and tinier bubbles. Of course, the pump must be large enough to create enough pressure to draw enough air, or the whole project is jeopardized. This sometimes means a fairly pricey dedicated pump is needed. A pump that large can create more heat that will need to be dissipated. The venturi valve will also need to be cleaned frequently - not a difficult job, but it must be done or the amount of bubbles will be diminished.