NO2 / NO3 – Nitrites and nitrates in aquarium water.
Aquarists should pay attention to certain aspects of the nitrogen cycle taking place in water because, on the one hand, nitrogen compounds are extremely necessary for plants and fish, and on the other hand, they can have a strong toxic effect. Organic metabolic products, leftover food, dead plants, etc., are converted by nitrifying filter bacteria living in the substrate into ammonia (a poison), into nitrites (poisonous, toxic, and extremely dangerous to aquarium inhabitants), and then into relatively harmless nitrates (a poison, but only in large quantities, serving as an intermediate product), which are only partially absorbed by plants. Nitrates are significantly less toxic, although in high concentrations they lead to reduced immunity, poor coloration, and subsequently the death of the fish.
The more decaying old plant leaves, fish excrement, and uneaten fish food there are in the aquarium, the higher the level of nitrite [NO2] and ammonium [NH4] will be. Nitrification is the further decomposition of dissolved ammonia by special bacteria that oxidize toxic ammonia through nitrite [NО2] into nitrate [NО3]. These “specialized” bacteria (Nitrosomonas and Nitrosococcus) first convert ammonia into nitrite, and then another group of bacteria (Nitrobakter and Nitrocystis) ensures the oxidation of nitrite into nitrate. These bacteria consume oxygen in the process of their activity, and when oxygen is deficient, the process slows down and the water becomes enriched with ammonia and nitrite. For them to turn into nitrates, the body of water must have good oxygen conditions.
In the top layer of the aquarium substrate, where oxygen is abundant, aerobic bacteria settle to process ammonium into nitrate. However, just a few centimeters deep into the substrate, there is already insufficient oxygen for nitrification to occur. Another type of bacteria begins to develop here – anaerobic bacteria, those that live without oxygen. Anaerobic bacteria, which do not need oxygen in the water, can come into action, turning nitrate into nitrite and ammonia. The nitrates formed in the first part of the nitrogen cycle are by no means the final product of ammonia decomposition. They are used by anaerobic, denitrifying bacteria to extract oxygen. Some of the nitrates are converted back into nitrites by anaerobic bacteria, and these are used by denitrifying anaerobic bacteria, oxidizing them to nitrogen. Denitrifying bacteria complete the nitrogen cycle process by breaking down the compounds into nitrogen dioxide and, ultimately, into its gaseous form.
The conversion of ammonia NH3 —> nitrite NO2 —> nitrate NO3 is called the process of nitrification. The conversion of nitrate —> nitrite —> nitrogen is called denitrification. Denitrification occurs only in anaerobic, oxygen-deficient areas of the substrate, which are usually located below its surface.
Nitrites are formed during the oxidation of ammonia salts into nitric acid salts. Bacteria convert ammonia and ammonium into nitrite; a concentration of 0.1 mg/l is acceptable, but no more than 0.2 mg/l, as it will become dangerous to fish. Then bacteria come into play, converting nitrite into nitrate, which is dangerous to fish only at high concentrations – from 50 mg/l, though this varies greatly for different species. It should be noted, however, that bacteria oxidize ammonia and nitrites only into nitrates. In nature, these bacteria colonize the top layer of the substrate (where oxygen is present) and other surfaces, but in an aquarium we have significantly less surface area for bacterial colonization per fish than in nature, which is why a biological filter is needed.
If the biofiltration process is disrupted, the amount of nitrite increases, which can lead to fish death. In aquariums with a large number of plants and well-functioning biofiltration, an excess of nitrites usually does not occur. Nitrite is almost as toxic as ammonia, and therefore the nitrite content must also be kept under control! An additional sign indicating a significant content of organics and its decay products in the water is the formation of persistent foam during water aeration (under normal conditions, air bubbles burst in 2-3 seconds). Special test kits are used to accurately determine the concentration of nitrogen compounds. The aquarium owner must take care of removing all harmful substances and metabolic products.
Nitrate is the final product of nitrification. Nitrates are a nutrient for plants, but unfortunately, they stimulate algal growth. Although it is theoretically possible to achieve 
an equilibrium where all nitrates formed from fish waste are consumed by plants, in reality, this is practically unachievable. Such a “perfect” aquarium must necessarily have very few fish, and for most aquarists, this is far from an ideal option. Since plants are unable to use the entire amount of nitrogen compounds as nutrients, and bacteria cannot cope with all the waste, their concentration will constantly increase. Nitrates themselves have low toxicity, but if their concentration is not monitored, a reverse transition of nitrates into nitrites will begin. In high concentrations, they lead to reduced immunity, poor coloration, and subsequently the death of the fish. This is one of the most important nutrient elements for algae.
In a nitrate-rich tank, lower vegetation appears almost inevitably. On the other hand, timely removal of nitrates from the water leads to a decrease in the amount of algae – they simply starve to death. In the absence of plants and with fish overstocking, an excessive accumulation of nitrates occurs, which in turn leads to the conversion of nitrates into nitrites. Rapid aquatic plant growth and regular water changes slow down the rate of nitrate accumulation. It must be remembered that nitrates, the final product of organic decay, are no longer removed from the water by themselves, but gradually accumulate. Therefore, biofiltration by no means replaces water changes. Water changes are mandatory regardless of what biofilters are installed.
A high nitrate level is especially critical for discus and other inhabitants of the Amazon basin. If detected using a test, you need to start changing the water. Regular water changes are the cheapest, safest, and most effective way to keep nitrate concentrations within reasonable limits. With each partial water change, nitrates are removed along with the old water. But one should not conclude from this that tap water contains no nitrates at all. After cleaning the filters and changing the water, the pH may increase noticeably. As a result, ammonia and nitrates will affect the fish simultaneously. A water change causes the fish to swim sluggishly near the water surface an hour or two later.
Here is what happens: the water change reduced the nitrate concentration by only one-third, but the ammonia concentration increased several times over. It entered the aquarium with the fresh water and was excreted by the fish. The performance of the biofilter drops over time. This happens due to the filter element clogging with debris. Therefore, it needs to be rinsed periodically. Washed biofilters noticeably lose efficiency and do not oxidize ammonia at the required rate. All this means that without the aquarist’s intervention, the nitrate level in the water will gradually rise and slowly but surely cause health problems for the fish. Nitrate can be removed from water using ion-exchange resins.
After setting up a new aquarium, the nitrite level may remain high until the end of the first month, after which it normalizes. This process from the beginning until this point is called “cycling the aquarium”. Finally, the time will come when the bacteria converting nitrites into relatively harmless nitrates also form a sufficiently large population. Ammonia and nitrites are extremely toxic to fish, but if the nitrogen cycle proceeds correctly, these toxic substances will turn into relatively harmless nitrates before their concentration reaches a dangerous level. Only then will the aquarium finally become safe for fish. This entire process takes several weeks. Good biological filtration implies the removal of dissolved organic compounds from the water and the complete processing of all highly toxic nitrogen compounds present in the aquarium into low-toxicity nitrate. If the bacterial colony in the filter cannot cope with the biological load of the aquarium, toxic ammonia/ammonium and nitrites accumulate in the water, posing a threat of algal blooms and fish disease.
Different fish species have varying resistance to these poisons. It depends not only on the species of the animal, but also on its age. Young fish die the fastest in this situation. If new fish are introduced to an aquarium where the nitrate level is higher than the one they are accustomed to, the fish will experience nitrate shock, which can even lead to their death. Nitrites react with blood hemoglobin and prevent oxygen transport. External symptoms in both cases resemble signs of oxygen deficiency, but are also observed when water aeration is good.
Fish living in water with a high concentration of this nitrogen compound gradually develop pale gill coloration. This is followed by outbreaks of bacterial infections. Signs of nitrite poisoning include darkening of the fish’s coloration and changes in behavior. Fish lose their appetite and stand motionless in the corners of the aquarium (often nose into a corner), fins are clamped, breathing becomes rapid, movement coordination is impaired, scratching occurs, the body is slightly slimy, and if measures are not taken, the weakest individuals may die. In dead fish, the gills are dark and brownish. Even after returning to normal hydrochemistry following this cataclysm, the fish may remain ill for a long time. Gill filaments are often destroyed. Complex bacterial infections develop. The best treatment is Cifran with Metronidazole from a regular pharmacy. Dosages range from 1 g of Cifran per 100 l and 50 mg of Metronidazole per 100 l.
If these symptoms are detected, the water should be tested for nitrites, and if found, action must be taken. A partial water change must be performed urgently. In case of nitrite poisoning, do not change the aquarium water too drastically! The main mistake beginners make that leads to fish death is rapid water change. No more than 1/4 of the water volume should be changed at a time. It is better to do this in small doses to avoid shock from a sudden change in water composition. It is also a good idea to add a little conditioner to the water. The next change can be made in about 5-6 hours. Water changes will need to continue until the nitrite concentration drops to a safe level.
No filter can purify water from nitrates [NO3], which gradually accumulate, making systematic water changes of up to 30% mandatory. If the filter capacity is insufficient for a specific aquarium, no water change will prevent an algal invasion. Carbon does not remove nitrates and phosphates, which are one of the main causes of algae in an aquarium. On the contrary, poor-quality carbon will release phosphates. On the other hand, carbon will absorb fertilizers necessary for plants, such as iron. By making the water crystal clear, carbon masks the need for a water change – the aquarist will think the water doesn’t need changing while nitrates are off the charts.
No matter what the filter looks like, it cannot replace regular partial water changes.
Cl – Chlorine in aquarium water.
Ammonium and ammonia in aquarium water. Ratio of ammonia and ammonium in water. 
Last changes and additions made on June 30, 2012.
