Bacteriological nitrification is the most practical method for the removal of ammonia from closed aquaculture systems and it is commonly achieved by setting of sand and gravel bio-filter through which water is allowed to circulate. The ammonia oxidisers are placed under five genera, Nitrosomonas, Nitrosovibrio, Nitrosococcus, Nitrolobus and Nitrospira, and nitrite oxidisers under three genera, Nitrobacter, Nitrococcus and Nitrospira. There are also some heterotrophic nitrifiers that produce only low levels of nitrite and nitrate and often use organic sources of nitrogen rather than ammonia or nitrite. Nitrifiers in contaminated cultures have been demonstrated to nitrify more efficiently. Nitrification not only produces nitrate but also alters the pH slightly towards the acidic range, facilitating the availability of soluble materials (Ayyappan and Mishra 2003).