Aquaria Information from



Let's preface these notes 

by saying two things: 

1) Understanding biological filtration is THE most important piece of knowledge in being able to successfully keep tropical fish.

 2) Every person who keeps and displays tropical fish should understand this important concept and PRACTICE it! 

   SO- take advantage of this opportunity! Read this article, then make copies of it for friends who have aquariums in their home or place of work 

   Biological filtration is simply the process where organic matter containing nitrogen is broken down and converted into less and less toxic forms of nitrogen. The process involves several stages, and begins with the introduction of nitrogen into the aquarium in the following ways: 

1) Urea and fecal matter produced by the fish. 

2) Protein from decomposing fish food. 

3) Protein from decomposing fish and/or plants. 

   It is equally important to know that Ammonia is a by-product in the first conversion. Ammonia is very toxic to fish, and the higher the PH, the more toxic the ammonia. It's true. The more alkaline the water, the more toxic the ammonia. This is why ammonia poisoning occurs with such frequency in brackish, African Cichlid, and especially salt water tanks, and why testing for ammonia is so critical. Our recommended PH of 6.8 - 7.0 for fresh water incorporates this important concept. In areas of the Northeast where tap water is hard and alkaline, you should understand this aspect of water quality, and how to cope with it. Ammonia is converted to nitrite by bacteria called nitrosomonas and others. These bacteria live primarily in your filtering medium: floss, bio balls, filter pads and in the undergravel filter. 

   While nitrosomonas are found throughout the aquarium - sticking to the glass and the gravel- most live in the filter medium, due to the great amount of surface area. 

   Nitrite, although less toxic than ammonia, can wipe out a tank if the level is high enough. Nitrite is converted to nitrate by friendly bacteria called nitrobacter, and others. Like the nitrosomona bacteria, these live primarily in the filter medium. High nitrite levels, particularly when long term, are detrimental to the aquarium environment and the fish. In the normal aquarium, nitrate is either removed from the aquarium by water changes or used by plants and algae as food. 

   The nitrifying bacteria require significant time to reproduce and establish themselves in a new aquarium. A brand new aquarium does not have sufficient nitrifying bacteria to support more than a few small fish, and this is especially important to the new hobbyist. 

   PATIENCE when adding new fish MUST be exercised to avoid deadly ammonia or nitrite levels. It normally takes anywhere from 4 to 8 weeks for enough nitrifying bacterial to develop. Test kits should be kept on hand by all hobbyists so they can monitor the ammonia and nitrite and nitrate (especially with marines) levels daily in newly set up aquariums. As the aquarium becomes older and better established, many fish can build up a tolerance to higher ammonia, nitrite and/or nitrate levels. Under these conditions, careful daily monitoring and control is necessary only when new fish are introduced. 

   We all want to be successful hobbyists who know how to care for our fish. Understanding the nitrogen cycle and practicing the principles of biological filtration is a step in that direction. The reward is a clear conscience, and an even clearer tank!

Return to Table of Contents