Again and again aquarists report that almost their entire fish stock dies within a short time. In such cases, new fish were practically always placed in the affected aquarium. Resistant or unknown strains of bacteria, overbred fish species, unrestrained use of antibiotics in farms, etc. are often cited as explanations. At the end of the day, attempts at explanations of this kind remain mere speculation without careful investigation. They often only serve to reassure the person concerned that they have not made any mistakes in their posture and have become the victim of an inevitable stroke of fate.
On closer inspection, these mass deaths can usually be traced back to avoidable causes and at least explained with known causes.
Mass extinctions within a few hours
Mass die-offs, in which many fish die within a few hours, can usually be traced back to poisoning. Nitrite poisoning, which can be traced back to incorrect care, is particularly common. Ammonia and ammonia poisoning are also caused by care errors. Basically, nitrite, ammonium and ammonia poisoning are poisonings that occur at different times in the nitrogen cycle and are caused by excessive organic pollution of the aquarium water. An undetectable nitrite value is therefore no guarantee for unpolluted water. In American forums and newsgroups, the nitrite as well as the ammonium or ammonia value is usually given in response to corresponding inquiries.
Rapid mass extinctions at high temperatures are often triggered by a lack of oxygen.
In homes with copper piping, copper poisoning is also a major cause of rapid mass deaths.
Mass extinctions within a few days
Causes of mass deaths within a few days are usually parasites or bacteria. Parasites are usually recognized quickly and therefore offer little room for the speculation mentioned at the beginning. In contrast, bacteria often cause unspecific symptoms or remain completely undetected, so that bacterial infections often cannot be clearly assigned to specific or known diseases.
Fish are constantly exposed to a variety of pathogens, especially bacteria, in the water. The burden of pathogens in fish is significantly higher than in land dwellers. This applies in particular to the fish in the aquarium, since aquarium water is almost always more heavily polluted than the water from the natural distribution areas of the fish.
However, the fish are not helpless in the face of this bacterial pressure. In addition to the body’s own immune system, the mucous membrane and the mucus produced there offer effective protection against bacteria. The slime can wash away bacteria and parasites. In addition, the slime contains antibacterial substances. Some substances work against very specific types of bacteria, other substances work against several types of bacteria at the same time. The protection against pathogens that are already known to the immune system is higher, but the immune system also protects to a certain extent against unknown pathogens. Therefore, most bacteria do not even penetrate to the fish surface, skin, gills, etc., but live in the mucus that protects the fish surface. There is also evidence that the microorganisms and bacteria that live in the mucus prevent new bacteria from settling there.
Healthy fish are thus able to withstand the constant attack of the many bacteria that are in the aquarium and still remain healthy. It is a common misconception to think that if there are no diseased fish in an aquarium, there are no pathogens present. As a rule, healthy fish also carry numerous pathogens. However, the pathogens cannot multiply to such an extent that an acute illness occurs.
Only when the conditions change in favor of the bacteria can the bacteria multiply on a massive scale and lead to disease in the fish. Such changes in conditions can hardly be avoided, especially when buying new fish. Even when catching the fish, microscopic damage occurs to the mucous layer or the fish skin. Added to this is the stress of catching, transporting and deploying. Existing and purchased fish are exposed to new competitors, disputes over feeding ranks, etc. All of these factors impair the fish’s defenses and offer numerous targets for bacteria and other pathogens to attack. Numerous bacteria settle on tiny, invisible injuries, where they find optimal conditions for multiplication.
Whether fish become ill in this situation, whether the existing fish or the purchased fish become ill first, depends on how healthy or weakened the respective fish are and which bacteria they were or are exposed to. These don’t have to be particularly dangerous or completely unknown species. It is much more likely that existing bacteria will suddenly receive better living conditions. Once these bacteria are suddenly able to multiply en masse, a deadly cycle can begin. Fish that are becoming weaker and weaker offer more and more bacteria better and better living conditions. Due to the high reproduction rates of bacteria, this can lead to the death of entire stocks within a short time.
Well-known bacterial diseases that can kill many fish of different species and entire stocks within a few days are e.g. e.g.:
These and other bacterial diseases often cannot be identified clearly and in good time. Treatment often only begins when the fish are already too badly damaged. It is often treated with unsuitable means.
In any case, proper preparedness and quarantine is the best, and often the only, way to avoid mass deaths, regardless of the cause. The more risk factors the fish are exposed to, the more likely they are to become ill and die. Many seemingly mysterious mass extinctions would certainly be preventable or, if properly diagnosed, could be ended.
Not Bacterial diseases that can kill many fish of individual species within a few days are e.g. e.g.:
These diseases are usually easy to recognize based on the symptoms and can be treated successfully accordingly.
Mass extinctions within a few weeks
If many animals die over a period of more or less weeks, one can no longer speak of typical mass extinctions. Many diseases caused by bacteria and parasites can spread more or less quickly to the entire fish stock under the right conditions and lead to the death of many fish.