How to diagnose a sick fish.
A healthy fish’s body has a number of protective mechanisms that prevent pathogenic bacteria from entering and are aimed at destroying pathogens that have penetrated the body. The mucus covering the fish’s body is characterized by antibacterial properties. However, in certain cases, for example, when the body’s immunity is weakened, the development of a particular disease may be observed. Changes in appearance or behavior demonstrated by a sick fish are called signs of illness. Many of these are non-specific – in other words, they are simply signs that the fish is sick, but they are not necessarily symptoms of any specific disease.
Diagnosing specific diseases is generally a process of logical reasoning. This is guided by a combination of signs demonstrated by the fish, 
data obtained from observing environmental factors, and the background history of the fish, the aquarium, and its other inhabitants. For example, if new items appeared in the aquarium and after that one or more fish became sick and developed non-specific signs, then their illness is most likely the result of deteriorating water quality. If external circumstances remained unchanged, except that a flowerpot from the garden was placed in the aquarium, and all the fish became sick, then it can be concluded that the fish were likely poisoned by pesticide residues from this pot. If new fish were introduced into the aquarium instead of a flowerpot, then there is a high probability that the problem lies with pathogens or parasites. All causes of diseases require different treatments. Yet in each case, the signs shown by the fish may be very similar.
The presence of a disease can only be determined when symptoms appear as evidence of the disease’s development, but a precise diagnosis can only be made by taking mucus scrapings from the fish. Most often, while the species identification of the bacterial pathogen is underway, it grows, and the disease in the aquarium spreads so extensively throughout the entire aquarium setup that it becomes too late to do anything. Usually, we rely on organoleptic signs, behavior patterns, color changes (paleness, appearance of mucous deposits, redness, various ulcers, erosions, necrotic phenomena). All these signs, unfortunately, appear at a time when the disease has already started and progressed into an active form. For this reason, it is very important to perform early diagnostics to detect carriage or the incubation period of an infection.
In a group of such sick fish, there will typically be fish with signs of illness and fish in which external changes have not yet manifested. Visually, one can determine with a certain degree of probability the nature of the disease: whether it is bacterial, fungal, or parasitic, although very often they occur simultaneously. For example, parasitic diseases very quickly lead to a weakening of immunity, which contributes to the development of a bacterial infection often accompanied by ulceration. Fungi settle on these ulcerated lesions as a secondary infection, and the toxins they release prevent wound healing. Thus, we have a whole bouquet of diseases occurring simultaneously in the same fish. Diagnosis is also complicated by the fact that the symptoms of many diseases are very similar, especially in the early stages of development.
If histological methods are used to examine the integuments of fish that do not yet have visible pathology, we find pathology at the cellular level, meaning the disease is in its latent period. If impression smears are taken from such fish and stained with special dyes that reveal bacteria, we discover a large accumulation of the same bacteria in the smears that we isolate from areas of the fish’s skin with visible pathology. The number of bacteria in different parts of the fish’s body will vary and depends on the species of the pathogens. This phenomenon occurs because the epithelial cover on the surface of the fish’s body consists of different types of cells, and the ratio of different cells varies across different parts of the body.
The mucous secretion produced by skin cells varies and mainly serves two important functions. The first is protection against bacterial infections, and the second function is “damping”, i.e., a lubricant that allows fish to glide in the water, reducing water resistance and dampening turbulent vortices that arise when fish swim, thereby reducing swimming speed. As the mucus moves away from the cells, it loses its bactericidal properties and becomes a good substrate for bacteria, especially pathogens that secrete enzymes blocking the protective function of mucus and cells. It is in such places that pathological foci subsequently form, which we see and use to guide our delayed diagnosis. However, a number of disease agents enter the body through the mouth and digestive tract, and then the infection affects the entire body; nevertheless, we observe external pathology, although not in all bacterial-fungal infections.
It must be understood that the same disease can cause different behavioral responses in various species of fish. However, both unusual behavior and a changed appearance 
of the fish indicate that it is feeling unwell and sick. There are infectious diseases from which some fish species will die out completely, while these pathogens will have no effect on other fish. In principle, it is impossible to determine with the naked eye which pathogen has affected your fish. An accurate diagnosis can only be made after a detailed examination of a sample under a microscope. If diagnosing based on external signs of illness is impossible, samples will have to be taken for laboratory analysis. To clarify the situation, either aquarium water samples (useful if poisoning is suspected, but the cause is unclear) or biological material obtained during a fish necropsy may be required. In both cases, the analyses must be performed with the help of a veterinarian.
It must be admitted that it is not always possible to accurately diagnose and determine a specific fish disease. Many diseases share similar symptoms, but their treatments can differ significantly. In such situations, the measures taken will depend on the circumstances. There is no point in treating an unknown disease by randomly applying chemical medications in the hope of accidentally hitting the right cure. Many medications are toxic to fish to varying degrees and can cause aggravation. Combining different chemical preparations (when different drugs are applied sequentially without removing the traces of previous treatment via partial water replacement) is a very effective way to kill fish through poisoning.
If only one fish is unwell, the cause may be a non-contagious disease unrelated to the environment, such as a tumor or old age. If all other fish show no signs of illness and environmental conditions appear optimal, then usually nothing needs to be done. However, observe the sick fish so as not to miss additional signs that may appear. If this fish dies, a necropsy can be performed out of curiosity rather than necessity. On the other hand, if within a short period several fish fall ill, or all fish or individual specimens fall ill one after another with the exact same signs, and a diagnosis cannot be made, then options such as water analysis and post-mortem examination should be considered.
If tank mates constantly bully a fish and it cannot compete with them for food or experiences stress in the company of other fish for other reasons, it should be moved to a peaceful and quiet hospital aquarium. But since such a move causes stress, it should be done only when truly necessary. If the sick fish was recently purchased, timely isolation is the wisest decision. This measure may help prevent the spread of an infectious disease.
Most aquarists assume that any health problems in fish are caused exclusively by pathogens or parasites. In reality, most diseases stem from a poor living environment. Therefore, one should improve the fishes’ living conditions without resorting to medications. Even if it is established that the fish are afflicted with an infectious disease, unfavorable environmental conditions are still what trigger the outbreak. If the signs of illness are ambiguous, pay attention to the details of your aquarium maintenance and other situational specifics; this often helps rule out a large number of items from the list of possible diseases.
Know what you are treating. The first thing to start with is establishing an accurate diagnosis for the sick fish. Without this, proper treatment will be like shooting sparrows with a cannon. If you ask someone for advice, make sure of their competence. If you search for advice on the internet, do not be lazy to use a search engine and spend time reading 10-20 different sources, and most importantly, pay attention to how following certain recommendations turned out.
Health to you and your fish!
