1 - General

All life forms require nutrient components that they use to obtain energy and also nutrient components that are used for constructive metabolism. Constructive metabolism refers to growth through the formation of biomass.

The growth of all life forms in the garden pond depends on various factors. If one of these factors is not available in sufficient supply then growth will be limited. This is referred to as the ‘limiting factor'. The limiting factor can vary for different organisms in the garden pond.

  • Fish and other animals need high-quality, energy-rich, and nutrient-rich food (e.g. insects, plants, fish food, etc.) that at the same time is growth limiting.
  • Water plants need nutrients, (N and P) carbon dioxide, and light. They can meet their nutrient demand from the water or from the soil. In this case the nutrients nitrogen (N) and phosphate (P), carbon dioxide, or light are usually growth limiting.
  • Algae have the same basic needs that water plants have. However with the exception of string algae, algae can only utilize the nutrients dissolved in the water. In this case light and the nutrient content of the water are growth-limiting.
  • Microorganisms decompose dead organic biomass in the course of mineralisation. The nutrients can originate directly from the biomass or from the water. In this case energy content and availability of dead biomass, as well as availability of oxygen, are growth-limiting.

For metabolic processes, intermediate products and end products are formed, which are then in part dissipated in the water as nutrients. Nitrogen (ammonium or ammoniac, nitrate, nitrite) and phosphor (phosphate) are the most important nutrients.

The natural growth limitation of algae through the nutrient content of the water is nullified by the supply of additional nutrients such as fish food. This results in the known disadvantages described above.

2 - Ammonium and ammonia

Ammonium/ammonia is the first inorganic nitrogen compound that occurs when decomposing protein and it is secreted by fish via the gills. Ammonium/ammonia is also released through mineralisation of biomass. A constant low ammonium/ammonia content is important for the health of the fish in the pond as ammonia is highly toxic to fish.

Ammonium and ammonia have a balanced relationship.

NH4++OH-NH³+ H²O

This balance depends on the pH value of the water. If there is an increase in the pH value the balance point shifts toward toxic ammonia. For example at pH 7 the ratio of ammonium to ammoniac is 99:1. With an increase to pH 9 the ratio changes to 70:30. Increased ammonia/ammoniac content becomes more critical for garden pond fauna the higher the pH value becomes. In well-maintained garden ponds with adequate filtration capacity ammonium/ammonia cannot be detected or can only be detected in extremely small amounts! Any verification of ammonium/ammonia is an alarm signal and indicates that the pond is not adequately filtered biologically.

One way to detoxify ammonia/ammonium and nitrite involves microorganisms; this is called nitrification. The breakdown process is divided into two steps that are executed by different microorganisms.

The first step involves breaking ammonia/ammonium down to nitrite. This oxidation is performed by bacteria that are referred to as “first-order nitrificants“. In the second step nitrite is broken down by other microorganisms (the “second-order nitrificants) into nitrate. In both oxidation processes the bacteria withdraw the necessary oxygen from the water. The first part of the nitrification process is slower than the second because first-order nitrificants only grow slowly.

The most effective reduction of nitrite and ammonium/ammonia involves the use of starter cultures with supplemental nutrients, such as Biokick CWS paired with an adequate oxygen supply or through Oxytex CWS.

3 - Nitrite (NO²-)

Nitrite is formed through nitrification, which requires at least a water temperature of 10°C, by special microorganisms in the detoxification of ammonium/ammonia. (See Fig.). Nitrite concentration above 0.2 mg/l has a toxic effect on fish. In well-maintained garden ponds with adequate filtration or circulation nitrite cannot be detected or is only detected in very small amounts and always remains under the value of 0.2 mg/l! Any verification of nitrite is an alarm signal and indicates that the pond is not adequately filtered biologically.
If there is fish stock, measures to reduce nitrite, such as changing the water and adding filter starter, should be initiated without delay. Stop feeding the fish for a time. The nitrate values should be checked at regular intervals to prevent harm to pond inhabitants. If the nitrite values are too high fish get gill necrosis, which means the gills are no longer able to absorb oxygen, and thus there is a hazard of death by suffocation.

4 - Nitrate (NO³-)

Nitrate is the preliminary end product of protein decomposition and develops via the step-by-step breakdown of ammonium by nitrite. It occurs in the nitrification process through the breakdown capacity of the second-order nitrificants (microorganisms). As opposed to ammonia, nitrate and nitrite are not toxic to fish, thus they do not represent a direct threat to the fish stock.

Rather nitrate is a fertiliser that stimulates plant growth. Increasing nitrate content automatically means increased plant growth. The consequence is pond turbidity due to algae bloom. This means that the biological balance is disturbed. The dead algae sink to the floor where they are broken down by the microorganisms under high consumption of oxygen. This breakdown releases the nitrate that was previously constituted in the plant cell, which again induces increased algae growth. The process can only be interrupted if the microorganisms convert the nutrients into their own biomass, or into atmospheric nitrogen that is not available to plants.

Further processing of the nitrate to atmospheric nitrogen is handled by an additional group of bacteria, the denitrificants. Denitrification is the term for breakdown of nitrate via nitrite (nitrite remains bound and is not released) to gaseous nitrogen (Fig. 3). Gaseous nitrogen is chemically stable and is no longer available for plants and most algae. Through denitrification the cycle of nitrate production and utilisation is effectively interrupted. Denitrification occurs exclusively in low-oxygen environments.

Fig 3: Nitrogen cycle and decomposition of pollutants via Biokick CWS