Nitrogen is poorly soluble and, thus, not dangerous to fishes. Still in a small tank that is strongly aerated there is a possibility that the nitrogen bubbles to plug up the blood vessels when getting into circulatory system.
Nitrogen is a part of protein molecules and can be found in fish wastes, uneaten food, decaying parts of aquatic plants, dead snails and other debris. Heterotrophic bacteria break down this organic matter into amino acids and then - into ammonia and ammonia ions (ammonium).
Ammonia (NH3) is the most toxic compound of all nitrogen compounds. As little as 0.06 mg/l concentration is dangerous for small fishes; 0.2 mg/l concentration at long-term exposure is lethal.
Ammonium (NH4+) in contrast to ammonia is practically harmless. To a very must degree the pH-value of water determines, which of these substances forms in greater amount.
pH of water % of ammonium
(NH4+) % of ammonia (NH3)
6.9 100 0
7 99 1
8 96 4
9 75 | 25
| | | | | | | | | | | | | | |
Thus, if the pH in the tank is below 7.0, there is no danger the fish will be poisoned with ammonia. But when about 1/2 of volume is changed with water that has pH over 7, much ammonia ions can pass into ammonia and may cause the fish poisoning.
The bacteria called Nitrosomonas and Nitrosococcus take ammonia and ammonium to nitrite (NO2-). Its concentration should not exceed 1 mg/l. Otherwise it would be dangerous for fish. On the basis of nitrite presence in the tank the bacteria called Nitrobacter start their activity. They turn nitrite into nitrate (NO3-). This substance is dangerous only when the concentration goes beyond 80 mg/l. Ammonium and nitrate are a good supply for aquatic plants.
The process described above is called nitrification. The participant bacteria live mainly in the substrate and in the filtrated matter inside the biological filter. These bacteria need oxygen for their vital activity. When the water lacks oxygen the rate of nitrification process reduces and the concentrations of ammonia and nitrite grow up. Some anaerobic bacteria that don't need oxygen may come to operate and take nitrate to nitrite and ammonia.
Since water plants are not able to use the whole amount of nitrogen compounds and the bacteria cannot disintegrate all the wastes, the concentration of the latter gradually rises. Therefore the aquarist must keep to the tank care rules. The special attention should be paid if the tank water has the pH over 7.5.
Constant flow of fresh water allows maintain the contents of nitrogen compounds at a more or less stable level.
One can purchase special chemicals that allow determine concentration of various nitrogen compounds in water.
Translated by Tatiana Karpova (Moscow)
(MSU, Biology faculture, Dep. zoology and ecology).