Female customer watching fish in aquarium tank

Anaerobic bacteria in an aquarium – horror or standard?

If you ask a random aquarist if there are any places in their aquarium where there is no oxygen, most will certainly answer “no” right away without a second thought. For it is difficult to imagine that in such a relatively small environment, where the water is constantly mixed by a filter and often additionally mechanically oxygenated, it is possible to set apart a place where there is a perpetual war for the territory occupied by anaerobic bacteria. Why war? Why do we call it this way? Are they even necessary? I encourage you to read the article about anaerobic bacteria in an aquarium.

Liquid oxygen distributor

In the environment of both the entire planet and the home tank, we can find water everywhere. It is one of several necessary conditions of life. Not just because of the animals and plants that live in it. Equally important are the properties of the water itself as it is a great solvent for a lot of chemicals and gas molecules like oxygen. It has a high specific heat, so it accumulates energy well.

Is there no life without oxygen?

Every fan of science-fiction movies and books, thinking about life without oxygen, immediately has before his eyes the vision of lack of oxygen on the space station, and thus the death of his favorite hero. However, there are organisms much smaller than humans. It is impossible to see them with the naked eye. These organisms thrive in environments devoid of oxygen, cleverly obtaining it from the structures of chemical compounds.

Lake Tanganyika as a perfect example of anaerobic environment

It is perhaps Africa’s most famous body of water. It is 677 km long (the distance between Zakopane and Gdańsk), has an average width of 50 km and a total area of 32 900 km2. With a depth of up to 1 433 meters (almost 5 Eiffle Towers) and a specific terrain (which affects the winds blowing there), it is impossible to keep the water circulating. This means that there are layers in the lake where anaerobic conditions exist. During numerous studies of this environment, it has been noted that there is no accumulation of nitrogen. In a lake with such a rich fauna and huge volume, how is it possible for nature to thrive?

Research on BCC in Lake Tanganyika

In 2002, a group of scientists decided to check the so-called bacterial community composition (BCC in short). Previously, this type of study was conducted in shallow lakes, seas and oceans. However, as far as old deep lakes are concerned (Lake Tanganyika started to form about 12 million years ago), only Lake Baikal (the deepest lake on our planet) has been researched.

Studies on BCC have revealed that there are vertical zones arranged along gradients of temperature and oxygen concentration. As you will note in the graph below, this lake with an average depth of 150m contains water devoid of oxygen. It is therefore the world’s largest reservoir of anaerobic water by volume. The diversity of species, and thus adaptations to the prevailing conditions, among the bacteria at different depths suggests that a balance has developed in the lake which ensures the stability of the ecosystem. Stability, which is more or less consciously fought for by aquarists, also by advocates of “low-tech” type aquaria that use a layer of soil at the bottom of the tank.

What does it look like in Lake itself?

The terrain around Lake Tanganyika means that there are no winds capable of intense mixing of water layers. It is therefore the opposite of Lake Baikal. Because despite its greater depth, such mixing, and therefore such oxygenation, occurs throughout the volume. The changes with depth are ideally shown in the graph below from a 1999 research paper entitled Nitrogen Dynamics in Northern Lake Tanganyika. It has been redrawn for the purposes of this article due to the poor quality of the original.

Wykres Tanganika

By analyzing just this one graph, we can come to a great many conclusions that, on a micro scale, will also apply to home aquarium.

  1. We note that there is a low concentration of ammonium ion (NH4+) and nitrate(V) ions (NO3) in the surface zone due to their intensive uptake by phytoplankton.
  2. Ammonium ions remain low up to a depth of 150 m (even in light-free conditions). This has to do with the nitrification process, which oxidizes ammonium ions to nitrate in the presence of oxygen and limited light.
  3. Below a depth of 30 m, NO3 levels begin to increase due to reduced light. Thus, the phytoplankton ceases its activity. NO3 itself accumulates in water due to nitrification processes.
  4. The level of nitrates rises to a depth of about 75 m. Then their concentration starts to decrease due to the decrease of oxygen in the water. Anaerobic bacteria that are adapted to utilize the oxygen contained in the NO3 ion come into action.
  5. When the oxygen concentration in the water drops to zero, the nitrate(V) level also decreases dramatically. Because they become the only source of oxygen for the bacteria of the anaerobic environment. NO3 is denitrified to a molecule of N2 gas.
  6. As the amount of NO3 decreases, the concentration of ammonium NH4+ increases. At this depth there is no dissolved oxygen in the water, so nitrification is impossible. Therefore, the concentration of ammonium ion increases, followed by a process called mineralization of organic nitrogen, whereby it accumulates in the bottom sediments.

Anaerobic bacteria in the home aquarium

You may be surprised that there are anaerobic bacteria in every aquarium. You can come across them in a layer of gravel, sludge, a biological filter medium with the right porosity or a clogged filter. Everywhere denitrification is carried out on a small scale by diffusion of NO3 into the layer. Thanks to the presence of such bacteria, previously trapped metal ions are also released, such as iron, which I wrote about in the article Iron (Fe) in the aquarium.

Anaerobic bacteria under full control

While so far we have considered examples of anaerobic bacteria in natural conditions, produced without human intervention. There is nothing to prevent us from creating a place whereby we remove NO3 from the aquarium on an ongoing basis and under full control of anaerobic bacteria. For this we will need a nitrate reducer. It is a bucket filter (canister) shaped device, usually paired with a conductivity meter.

Water conductivity is measured over a wide range, from negative to positive values. When processes associated with aerobic conditions are present, the meter will always show a positive value, such as +100 mV. Under anaerobic conditions, conductivity takes on a negative value, such as -100 mV. It is therefore a reliable way to instantly determine the conditions we are dealing with.

Water flows very slowly through the reducer. If it is paired with a meter, the controller runs the pump depending on the water conductivity readings. Such a small flow allows oxygen to be consumed in the reactions occurring in the water per unit volume before it enters the reactor, where oxygen cannot enter.

A large number of protons and electrons are required to carry out the denitrification process. Therefore, operating a nitrate reducer involves “feeding” the bacteria. Professional reducers use biodegradable polymers for this purpose. The salt of acetic acid is an equally good source of protons and electrons. The reaction discussed sums up as follows:

5CH3COO + 8NO3 + 3H+ → 10HCO3 + 4N2 + 4H2O

From the diagram above, anyone with an interest in marine aquariums already knows that this forms the basis of the VM and VSVM methods. I might develop this topic further in the future.

The use of a nitrate reducer obviously involves the possibility of some technical problems. However, this is a topic for another article.

Let’s not disturb them

As we already know, anaerobic bacteria in a home aquarium not only exist, but they are also very necessary. They reduce NO3 levels and release elements essential for plant functioning into the water. They also break down dead organic matter. They effectively make life cycle close.

Certainly many beginner aquarists thinking that “sterile” conditions guaranteed by frequent filter cleaning and bottom de-sludging are the right action are painfully convinced that gigantic amounts of nitrate ions, among others, are released into the water during those activities. These ions waited quietly deposited in the substrate for their turn to be sacrificed to the anaerobic bacteria, so that they could live and make our tank maintain stable parameters, and the plants and animals enjoy their appearance.

Michał Wyskiel


N. Brion, E. Nzeyimana, L. Goeyens D. Nahimana and W. Baeyens Nitrogen Dynamics in Northern Lake Tanganyika (1999)

A. De Wever, K. Muylaert, K. Van der Gucht, S. Pirlot, Ch. Cocquyt, Jean-Pierre Descy, Pierre-Denis Plisnier, W. Vyverman Bacterial Community Composition in Lake Tanganyika: Vertical and Horizontal Heterogeneity (2002)

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