dirty aquarium

Hydrogen sulfide poisoning symptoms in fish


When the fish are over-fed at the bottom of the aquarium, the anaerobic environment is formed from the remains of food, detritus, and feces, in which nitrates are converted to nitrogen. But what happens if, as a result of anaerobic decomposition of organic compounds, nitrates will no longer remain? Such a development would be dangerous for the fish! Anaerobic bacteria will continue to decompose organic compounds in an oxygen-free environment. At the same time, those proteins and amino acids that contain sulfur will also undergo destruction. Sulfur will be reduced to hydrogen sulfide, colorless, smelling of rotten eggs, highly toxic, and highly soluble gas in water.


In the case of hydrogen sulfide poisoning, fish suffer from suffocation caused by an acute lack of oxygen. They stick to the surface of the water and gasp for air. The fish are choking. At autopsy, the violet color of the gills is especially striking.

Hydrogen sulfide is very toxic and damages primarily blood. Hydrogen sulfide binds in hemoglobin sites intended for oxygen and thereby interferes with its absorption.


If the presence of H2S(Hydrogen sulfide) is really detected in the water, you will have to empty the entire aquarium with the filter, thoroughly clean, and rinse everything to eliminate organic pollution. When cleaning the bottom, anaerobic sections in the soil are detected by the black color of sand (iron sulfide) and the unpleasant smell of rotten eggs (hydrogen sulfide).


Preventive measures to prevent the formation of hydrogen sulfide, which poison the aquatic environment of the aquarium, are quite simple. It is important to give the fish as much food as they can eat in a few minutes. The food should not settle to the bottom and decompose there. Feces, detritus, and residual food must be removed in a timely manner from the bottom (from the surface of the soil) by suction or by other means to prevent excessive oxygen consumption. In such a (clean) aquarium, dissolved metabolic products (ammonia and amino acids) are immediately oxidized to nitrates. Nitrates are converted to elemental nitrogen by anaerobic decomposition at the bottom. The carbon dioxide released in this process is absorbed by the plants, and the harmless elemental nitrogen is removed by aeration. These measures result in healthy fish and lush aquatic vegetation.


To verify the fact that in the presence of nitrates in the anaerobic environment neither hydrogen sulfide nor methane is formed can be done in one small experiment. Take two ordinary glass cups and pour so much dry food into both of them to completely cover the bottom. Then fill both cups three-quarters of aquarium sand or gravel. Next, fill one glass to the top with normal tap water, and the other with water, in which two full teaspoons of sodium nitrate will be dissolved. In a few days, you will see and smell the difference! In a glass without nitrates – a terrible stench. In a glass with nitrates in the sand, gas bubbles can be observed, there is no foul smell.