Nitrate is a key element in all aquaponic systems as it’s formation, and assimilation, is vital for the health of both fish and plants. Chemically speaking, nitrate exists as NO3–. This means that one nitrogen atom is coupled to three nitrogen atoms. It also has a single negative charge. that influences how it is transported. It is the most oxidised state of nitrogen. This can be important as it can represent a potential source of oxygen under anaerobic conditions.
Why nitrate is important
The Nitrogen Cycle is fundamental to the operation of every aquaponic system. Nitrate plays a key role in this cycle and is important for the following reasons.
- Nitrogen is essential for the growth of plants and fish. It is a key constituent of amino acids, proteins and nucleic acids.
- Plants growth is limited more often by a defiency of nitrogen than any other plant nutrient.
- The formation of nitrate, through the process of nitrification, reduces the concentration of toxic ammonia that is continuously being excreted by the fish.
- Nitrogen is an important ingredient of chlorophyl and consequently, plays an essential role in photosynthesis.
Where does it come from?
Ultimately, nitrate in fish tanks is generated from the food that is fed to the fish. In particular it comes from the protein part of the fish food. Nitrate is not something produced directly by fish, rather an inevitable product of a successful biofilter. These filters work by converting ammonia, a direct waste product excreted via the gills, into nitrate via nitrite. It is essentially a two-step process.
- The conversion of ammonia to nitrite by ammonia oxidising bacteria. (AOB).
- The conversion of nitrite to nitrate by nitrite oxidising bacteria (NOB).
These compounds are all key players in the nitrogen cycle. Nitrate can both be produced from ammonia and nitrite and produce these compounds should the environmental conditions allow. It is an integral part of the Nitrogen Cycle or web as it is now more accurately described.
Denitrification is the process by which nitrate is reduced to gaseous nitrogen, N2. Nitrate can go in two separate directions as it travels through the Nitrogen Cycle.
- Assimilation to form plant proteins
- Denitrification to form gaseous nitrogen
Which direction it follows is largely determined by the presence, or absence, of dissolved oxygen. If the level is high, as required by fish, there tends to be an accumulation of nitrate. Conversely, if the level is low, as may be the case in the bottom of a grow bed, nitrate will be broken and reduced to nitrite. This will then be further reduced to gaseous nitrogen that subsequently escapes to the atmosphere, In all aquaponic systems, and most ponds, there is a relatively high level of oxygen and so nitrate levels overall tend to rise.
The reason biofiltration is such a successful technique is because nitrate is considerably less toxic to fish than either ammonia or nitrite. The Ornamental Aquatic Trade Association (OATA) recommend a maximum level of 50 mg/l above background levels of local tap water. This compares to 0.2 mg/l for nitrite and just 0.02 mg/l of ammonia. It is a matter for discussion whether or not even higher levels of nitrate , say 100 – 150 mg/l , are detrimental. A slight word of warning however. Low levels may not be directly toxic to fish such as koi but it is thought, by some, that nitrate can affect fish skin quality and colour.
Measuring and reporting nitrate
Nitrate in fish tanks is relatively easy to measure using widely available and inexpensive test kits. NT labs and API both manufacture popular kits. The fact that we are only interested in relatively high concentrations means that these kits are perfectly fine for most fish keepers.
In common with ammonia and nitrite, levels of nitrate can also be expressed in terms of their nitrogen content. i.e. NO3-N rather than just NO3–. A simple conversion factor needs to be used to correctly compare these two values. This conversion factor is calculated from the respective molecular weights of the elements that make up each compound. For nitrate a conversion factor of 4.4 should be used. So, a level of 10 mg/l of NO3-N is exactly the same as 44 mg/l of NO3. Quite a significant difference.
For ammonia the conversion factor is just 1.1 and nitrite it is 1.2. This obviously less of a difference and is often incorrectly ignored. It’s worth checking how your test kit report their results, i.e. NO3 or NO3-N and make adjustments of needed.
Remember these are chemical tests that should be run alongside careful observation. The visual symptoms of nitrogen deficiency means that it is relatively easy to spot any shortages. Symptoms include poor plant growth and leaves that are pale green or yellow. This is because the plants are unable to manufacture sufficient chlorophyl. Leaves in this condition are said to be chlorotic.
As implied by the OATA levels of nitrate in tap water may vary from one supply to another. The widespread use of nitrogenous fertilisers in agriculture has led to an increase in background nitrate levels of our tap water. This has been of concern for some time and moves continue to lower this level. High nitrate is thought to play a part in causing certain types of cancer.
Conclusions and recommendations
- Nitrate production in an aquatic system is directly related to the amount of food given to the fish. This factor can be used to calculate how many fish should be kept with how many plants.
- Nitrogen is an essential element for both plants and fish and s required in quite large quantities. With the correct balance of fish and plants it is unlikely that plants will show signs of deficiency.
- Fish ammonia excreted via the gills is an excellent source of nitrate thanks to nitrification.
- Fish poo forms a second source of this essential element. This time it is released through the process of ammonification. This represents a slower process but does ensure that supplies of nitrate can continue for a period even after feeding has stopped.
- Nitrate in fish tanks is largely non-toxic, even above 200 mg/l. Expect levels of around 10 – 100 mg/l.
I hope this has given a better insight into the role of nitrate in a fish tank. Remember it is relatively easy to main a healthy balance of nutrient generation and uptake. Levels are not critical and can usually be monitored by the careful observation of fish behaviour and plant growth.