Filters are basically devices that remove unwanted things and that is exactly their purpose in a marine tank. Water quality is extremely important for marine organisms. They live in the water and toxins and other impurities must be removed or at least converted into something less toxic.

There are three basic forms of filtration that can be employed in a marine tank:

Of these, biological is the most important and unless the aquarium has a flow through supply of natural seawater it will not function without some form of biological filtration. Chemical filtration can be used on occasion but is not required for most of the time. There is one form of mechanical filtration which is nearly as important as biological filtration but other forms should be avoided.

Biological filtration

As the name suggests, biological filtration uses organisms to remove substances from the water. In most cases the organisms are bacteria, but other organisms may also function as filters, including algae and even corals.

Central to the role of biological filters is the nitrogen cycle and the removal or conversion of toxic ammonia and nitrite. Biological filters provide suitable substrates for the organisms, predominately bacteria, that form part of this cycle. The substrates can be provided in a number of ways and some biological filters cover different aspects of the nitrogen cycle. Selection of appropriate biological filters for an aquarium should be based on the filters capabilities and the requirements of the organisms to be kept. Consideration should also be given to setting up and maintaining the filter.

Over the years, new forms biological filtration have replaced old forms, but the basic principle of providing a large surface area for bacteria has stayed the same. While some of the older methods are no longer in vogue, it doesn't mean that they won't work in specific situations. The generally available forms of biological filtration as as follows:

These forms of biological filtration can be combined with each other or with other forms of filtration as required.

Chemical filtration

Mechanical filtration


Last updated: November 3, 2002