Biological filtration is the critical filtration component in every aquarium.
Whether the biological filter is live rock, trickle media, a sponge or any other substrate, the biological oxidation of ammonia to nitrite and then to nitrate is necessary to keep these substances from reaching toxic concentrations that will kill the inhabitants of our aquaria. It has been a paradigm in biology and the aquarium hobby that there are two bacteria responsible for nitrification. The first, called Nitrosomonas europaea , oxidizes ammonia to nitrite, while the second, Nitrobacter winogradskyi , oxidizes nitrite to nitrate. These organisms are called nitrifiers and are classified as belonging to the same family of bacteria. However, recent work on the phylogenetics of these organisms and their close relatives has shown that this classification is wrong and needs to be revised.