One of the first recorded instances in history of marine aquatic life being kept in just about any sort of a closed system (aquarium by today's definition) was thousands of years ago when the pharaohs in Egypt had large enclosures built for keeping saltwater fish and invertebrates for the pharaoh's viewing pleasure. Those ancient aquariums, by all accounts, weren't exactly a roaring success. At that time, absolutely nothing was known about what it takes to make a closed system work. Nothing was known about the ammonia-nitrite-nitrate conversion and what caused it to happen.
Many, many years later, we now understand that it is simple, specific bacteria which digests toxic ammonia and convert it into less toxic nitrate, and another bacteria then converts the nitrites into nitrates. Nature has done it for billions of years, which allows life forms to live in the oceans, lakes, ponds and rivers. The bacteria which detoxify your aquarium water are the same as the ones in nature that keep the oceans from turning into a toxic soup.
The biological filter in an aquarium is nothing more than a place for bacteria to grow. Any surface in an aquarium which comes in contact with the nitrosoma bacteria that you created when you cycled the tank is part of your biological filter. Bacteria require food (ammonia in this case) in order to grow and multiply. The larger the bacteria population in your aquarium, the more ammonia can be processed and detoxified. The water in your aquarium contains bacteria which it circulates throughout your system. These bacteria attach themselves to and grow on any surface area it comes in contact with. The larger the surface area, the more bacteria will be able to reside in your tank.
The type of substrate you choose for your tank makes a great difference in the amount of surface area that is available for bacteria to reside on. Choosing your biological filter material is important when setting up your filtration system.
The efficiency (strength) of a biological filter is determined by its surface area. Some filter mediums are more efficient than others. Aquarium glass surface area aside, here are the most popular biological filtration methods:
- Canister-style filters come in many different styles and sizes and can also be multi-functional. Canister filters contain a number of chambers which can hold a variety of materials which perform a wide variety of functions. One of these functions is to hold a quantity of highly porous material (i.e. ceramic rings, bio-balls) to support bacteria growth.
- Live Rock/Berlin Systems are still the filter system of choice for many saltwater aquarium purists. The Berlin Systems consisted of a tank with a quantity of live rock (the biological filter platform), a protein skimmer (removes a large portion of the proteins before they have to be processed by the biological filter) and metal halide lighting. Since the invention of the Berlin System, the more efficient LED lighting systems have replaced the metal halides as the preferred lighting source.
- Live sand/Jaubert systems are also very popular with many reef aquarists. This innovative filtration system consists of a deep sand bed, a plenum, and a protein skimmer. Each of these components is an integral part of a complete biological filter, converting ammonia to nitrite, which is converted to nitrate (via aerobic bacteria), which is in turn converted to nitrogen (via anaerobic bacteria).
- Undergravel filters are among the first biological (and mechanical) filters created for saltwater aquarium hobbyists. There is still a great debate about the creation of excess nitrates.
- Wet/Dry Trickle Filters were the next step after the undergravel filter to dramatically improve efficiency. Wet/dry filters consist of running aquarium water over a biological filter material. An added benefit of the wet/dry system is that gas exchange between the atmosphere and the aquarium water is greatly increased.
Knowing how each biological filter works and what it requires will make choosing a filtration system mush easier.