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What Is The Nitrogen Cycle In Tropical Fish Aquarium?

Updated on March 15, 2014

Fish and plants in our aquarium should have a normal development, therefore the quality of water should be comparable to the water of the environment where they come from.

The success or failure of our aquarium, mainly depends on the quality of water. In the water of an aquarium several processes occur.

For water to be maintained in conditions that allow the development of fish and plants, a biological balance should be maintained between organic waste and bacteria.

This balance is maintained by one of the most important processes that occur in the aquarium, the nitrogen cycle.

For proper nutrition, plants need phosphates, but do not consume all the phosphate present in the water. In small amounts, the excess of phosphates is not harmful.

The pH level is one of the water parameters of most concern to the aquarist, and of course they have reasons to worry.

The pH level indicates the acidity or alkalinity of water, and it is important that an appropriate value for the species living in the aquarium is maintained.

Oxygen is necessary for plants and fish to carry out their vital processes. It is important to maintain a homogeneous level of dissolved oxygen in the water.

Nitrogen Cycle
Nitrogen Cycle | Source

The Nitrogen Cycle

Inside the aquarium organic wastes are produced, such as fish excrement, food waste and dead plants. The nitrifying bacteria that are present in the aquarium carry out the biodegradation process of organic matter.

The biodegradation produce nitrogen compounds, which is called the nitrogen cycle. From the organic matter, the bacteria produce ammonia (NH3) and ammonium (NH4).

Both compounds are toxic, the appearance of one or other compound depends on the pH:

  • Acidic water have higher amounts of ammonium
  • Alkali water have higher amounts of ammonia.

From the ammonia/ammonium the nitrite (NO2) is produced, which is also a toxic compound. This is dependent on the acidity of the water. The final product of the nitrogen cycle are nitrates (NO3). Nitrate is not very toxic in small quantities.

If the nitrogen cycle in the aquarium is being carried out properly, the concentrations of each compound when making a measurement must be:

milligrams per liter
Ammonia / Ammonium
10 to 30

Nitrates are necessary for plant development, so the concentration must never be zero.


The degradation of organic matter also produces phosphates. Phosphates can also enter the aquarium through the water exchange, and through the food.

Phosphate is a nutrient for plants and must be present in the water. The appropriate proportion of phosphates in water is estimated 0.1 to 0.2 milligrams per liter (or ppm).

If an excess of phosphate is produced so that the plants can't consume it, algae will proliferate.

Phosphate in ppm = Phosphorus in ppb * 3.066 / 1000
Hanna Ultra Low Range Phosporus Checker


pH is the measure of acidity of the water, specifically is the concentration of hydrogen ions (protons) in water (Pondus Hidrogenii in latin ).

It is expressed as a value without units in logarithmic scale of 1 to 14 (each jump of one unit is equivalent to 10 more times).The pH of 7 is established as neutral.

The values greater than 7 are said to be alkaline or basic and values less than 7 are said to be acidic.

Not all species require the same pH, nor a universal pH exist.

If the pH is too low for the species in the aquarium, there is a problem of acidosis. If is too high, there is a problem of alkalosis.


The oxygen in the atmosphere is present in a proportion of 20% and penetrates the water by the osmosis phenomenon, ie, the pressure difference between the atmosphere and the water.

The oxygen from the atmosphere penetrates into the water only 1 to 2 inches below the surface.

In order for the dissolved oxygen concentration remains relatively constant throughout the volume of aquarium water, the need to aerate the water arises.

The aerator systems does not oxygenate the water by themselves, the oxygenating action is based on the agitation that occurs in water.

This agitation causes a circular-rotational movement of water, putting all the mass of liquid in contact with the atmospheric air, and is at the surface where the oxygen penetrates.

Problems in the aquarium

A fish tank in good working order, is a system in equilibrium. The fish, the plants and bacteria interact with each other to maintain a favorable environment. This balance is not achieved in the overnight, it takes time and maintenance.

A newly assembled aquarium is not able to host occupants. Any new aquarium needs a period of maturation during which nitrifying bacteria grow and settle on the substrate and on the filter. Bacteria are necessary to produce the nitrogen cycle.

Once the tank is ready, it should not be overcrowded. The fish needs some space to develop. If the fish are crammed in the tank, get stressed and sick very easily.

Overcrowding also produces a biological imbalance. If there is excessive organic matter (more waste of fish, more food scraps) the nitrogen cycle is altered. The amount of phosphate also increases.

Besides avoiding overcrowding, put only compatible species in the aquarium. Do not mix aggressive fish with peaceful fish.


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