In streams, rivers, and oceans, fish find places where the environment is right for them to live, eat and breed. Since the temperatures and conditions of streams and rivers fluctuate seasonally, freshwater fish have to move a great deal. By contrast, saltwater fish have it easy. Although they are generally far more sensitive to water quality than freshwater fish, the ocean environment is relatively stable. The keeping of either kind of fish in captivity, however, relies upon a constant and reliable filtration system.
Most average aquariums don't need more than one filter to handle cleaning the water. Still, many people opt for more than one filter, if for nothing other than backup. After all, if you have come this far, why skimp on one of the most important systems of your entire tank set-up?
Three types of filtration are necessary for a typical tank: biological, mechanical and chemical. Although some filters are "dedicated" to performing a specific filtration task, most are multifunctional and can handle several - if not all - requirements. Though all types of filtration are necessary to keeping a healthy tank, experienced aquarists will agree that the biological process is the most crucial.
Biological filtration is a process driven by bacteria that consume the liquid waste your fish produce. This process is generally known as nitrification. Fish excrete ammonia, which is toxic to them. In a healthy marine system, one kind of bacteria converts the harmful ammonia into nitrites (Nitrosomonas) and another converts nitrites to nitrates (Nitrobacter). Nitrates cannot be easily removed from the water, which is the main reasons that those regular (weekly to bi-weekly) water exchanges are a must in a saltwater system.
These bacteria grow naturally in the coral rock, gravel, dolomite, filter media and just about any surface in your aquarium, so most hobby aquarists won't have a separate biofilter. But sometimes, especially in a very "crowded" or sensitive system like a coral reef, hobby aquarists will not rely on only the bacteria floating around in the system but dedicate an entire filter to this process. The filter-medium, commonly called bioballs, is made up of pieces of plastic that provide extra surface area for fostering bacterial growth. Biological filters must be cleaned, but never with soaps, detergents, or other such chemicals since they will immediately destroy the culture. Rinse them off with plain saltwater.
Mechanical filtration is merely a filter that takes particles out of the water. The filter medium is commonly a woven or foam-like material that traps the organics, solid wastes and any other particles out of the water. This often includes the "good" bacteria, so too much mechanical filtration is possible. The water from your tank should run through the mechanical filter first.
Chemical filtration is the process by which dissolved organic compounds are removed from the water. This is generally done with activated carbon, which is cheap and usually easy to replace. Several years ago, aquarists were deep in debate on whether using a carbon filter is worthwhile, a discussion that some still cling to more out of habit or nostalgia than necessity. But just about every experienced saltwater aquarist today will not go without a carbon source because it does a great job.
When soluble organic compounds build up in your aquarium, it will turn your water yellowish. Besides making your pets appear as though they live in a thick L.A. smog, the accumulation of these organics will interfere with the normal growth and development of your fish and is toxic to most fry and invertebrates. Organics also add to the detritus that can clog your other filters and promote the growth of "bad" bacteria, which will add more waste products to your system.
Protein skimmers, also called foam fractionators, are not typically used for chemical filtration in an average freshwater tank, or even in a "fish-only" marine system. They are usually expensive and not a necessity. But for systems like a coral reef tank, most experienced saltwater aquarists highly recommend getting a filter of this kind to supplement any mechanical and biological systems. Protein skimmers are very effective at removing various organics like protein, phosphates and fatty acids. These often first appear as small "oil slicks" on the water's surface and then, as they accumulate, allow big foamy bubbles to linger. Protein skimmers essentially siphon the water through an aeration system, forcing the waste to rise in a foamy mass of slimy bubbles into the skimmer cup. This cup must be cleaned daily. Protein skimmers take a huge load off of the mechanical and biological filtration, so if you decide to get one, the water should run through it first instead of the mechanical unit.
Several varieties of filter designs are on the market and range widely in price. One of the most popular is undergravel filters (though many complain these are too difficult to clean because you need to "vacuum" your gravel). Canisters are popular because they are sturdy and reliable. These are located outside of the tank and are often designed to accept different types of media.
Check around before you decide on what to get and make sure you are aware of what sort of filtration suits your needs. Two to five times the tank volume per hour is a "normal" cycling for many filter systems, although keep in mind that some species, like seahorses, cannot handle the current this cycle-volume produces. Just make sure that your filters can handle your tank volume and that your pets can live with your decision. Although integrating another filter into your system later is usually an easy task, you ultimately want to avoid disturbing your system when things have become established.
Getting lazy is the most dangerous part of filters. If you don't clean your filters and exchange the carbon on a regular basis, it will promote the growth of the wrong bacteria. These "bad" bacteria produce hydrogen sulfide (rotten egg smell) and methane, both highly toxic to marine life. Clean all of your filters on a regular basis.