What Is A Biological Filter For A Saltwater Aquarium?

Beautiful freshwater planted aquarium

Thousands of years ago, the pharaohs of Egypt had large enclosures built for keeping saltwater fish and invertebrates for the pharaoh's viewing pleasure, and this was one of the first recorded instances in history of marine aquatic life being kept in just about any sort of closed system (aquarium by today's definition). Those antique tanks weren't exactly a smash hit, according to all accounts. Nothing was understood at the time about what it needed to make a closed system operate. There was no information regarding the ammonia-nitrite-nitrate conversion or what caused it.

Many years later, we now know that simple, particular bacteria eat poisonous ammonia and convert it to less hazardous nitrate, which is subsequently converted into nitrates by another bacteria. For billions of years, nature has done it, allowing life to thrive in the seas, lakes, ponds, and rivers. The microorganisms that purify your aquarium water are the same ones that preserve the oceans from becoming a poisonous mess in nature.

In an aquarium, the biological filter is just a space for bacteria to flourish. A biological filter is any surface in an aquarium that comes into touch with the nitrosoma bacteria you developed when you cycled the tank. Bacteria need food to thrive and replicate (in this case, ammonia). More ammonia can be digested and detoxified the higher the bacteria population in your aquarium. Your aquarium's water includes microorganisms that circulates throughout your system. These bacteria cling to and proliferate on any surface with which they come into contact. More bacteria will be able to live in your tank if the surface area is greater.

The type of substrate you use in your tank has a significant impact on the quantity of surface area available for bacteria to colonize. When it comes to setting up your filtration system, choosing the right biological filter material is crucial.

The surface area of a determines its effectiveness (strength). Some filter media perform better than others. Aside from aquarium glass, the following are the most common 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.
  • 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).
  • 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. 

Understanding how each biological filter works and what it requires can make selecting a filtration system much simpler.