Continued…
Prevention of diseases caused by poisoning.
Toxic substances can enter along with tap water – for example, chlorine, chloramine, and insecticides used to destroy bacteria and invertebrates present in drinking tap water. Heavy metals such as lead and copper are also sometimes present in tap water. A common cause of toxic substances entering the aquarium water is inappropriate decoration and equipment of the aquarium, especially homemade ones and those of unknown origin. Danger can also arise as a result of improper use of medications or from toxic substances entering the water.
Often, aquarium water is taken from wells, springs, or other underground sources. Such water may contain a large amount of iron; under the action of high concentrations of iron, the gill apparatus of fish is destroyed: the gill filaments become anemic and covered with coagulation mucus, leading to asphyxia. Therefore, if there is no other water source, well or spring water must be boiled. In this process, iron, calcium, and magnesium salts precipitate.
• Chlorine and chloramine (a mixture of chlorine and ammonia) are added to tap water as disinfectants. These substances are extremely toxic to fish, and their toxicity increases as the pH value decreases and the temperature rises. Their impact can be either acute, where all fish are affected immediately, or chronic, manifesting as a prolonged general poor state of health. For acute poisoning, a chlorine concentration of 0.2 to 0.3 mg/liter is sufficient. Acute toxicosis: shortly after adding fresh tap water to the aquarium, fish begin to behave as if trying to escape from something.
• Iron, zinc, lead, and copper are the most common metals that can cause poisoning. Metals can form toxic salts when present in salt 
or acidic water. The chemical composition of water affects the type of metal salts formed. In particular, the presence of calcium in water can reduce the toxicity of certain metals, while an acidic environment can, conversely, increase toxicity. As a result, some heavy metals are more toxic in soft acidic water, i.e., water with a low pH value and low calcium hardness. The level of toxicity may be determined by a mixture of metals (if any). Susceptibility to these toxins can vary among different species of fish and even among individual specimens. They cause aquatic organisms to secrete abundant mucus. Deprived of mucus, the animals become completely defenseless against a renewed onslaught of new bacilli. Before drawing water to add to the aquarium, open the cold water faucet for a few minutes to flush out stagnant water in the pipes. Avoid the misuse of medications containing copper. Aluminum pans or utensils made of other metals can cause contamination of peat extract and other infusions prepared in them.
• Use only equipment suitable for aquarium water. Stones or flower pots from the garden may be contaminated with chemicals used in gardening. Stones may contain toxic compounds. Many types of plastic release toxic substances when immersed in water. Therefore, you should use only those plastic items that are specially designed for aquariums or food products. Varnished wood can poison fish that have a habit of gnawing on wood – for example, some catfish and Uaru.
• Paints, varnishes, glues, and colorants are toxic unless specifically intended for aquarium use.
• Food products, if stored improperly, can sometimes lead to aflatoxin poisoning. In addition, the aquarist or family members can inadvertently introduce various toxins into the aquarium.
• Paint or chemical fumes, tobacco smoke, aerosol polish – all these can penetrate the water through its surface or through the air pump. When pest insects are exterminated in living quarters, toxic chemicals (malathion, chlorophos, hexachlorane, etc.) can enter the aquarium, causing fish poisoning. During such treatments, the aquarium must be thoroughly isolated.
• Soap, cleaning products, and other household chemicals can enter the aquarium along with equipment, decorative items, or on hands.
This list cannot be considered exhaustive – it is a list of the most common causes of toxicosis.
Prevention of diseases caused by hypoxia (oxygen starvation).
Hypoxia is a condition, often fatal, when an insufficient amount of oxygen is supplied to the fish’s tissues. The cause can be physiological (gill damage in most cases leads to restricted oxygen uptake) or environmental (insufficient oxygen content in the water). However, the gill damage itself is most often also caused by unfavorable environmental factors.
Fish oxygen requirements vary by species. Usually, they depend on the natural habitat of the fish. For example, fish from well-oxygenated waters generally tolerate low oxygen levels in the aquarium worse than fish from oxygen-poor natural environments. Such fish may experience fatal hypoxia due to seasonal temperature rises in the aquarium (e.g., during hot weather), as this leads to a decrease in the water’s ability to carry oxygen and simultaneously an increase in the fish’s oxygen demand. Well-fed fish require more oxygen than hungry ones.
Oxygen demand can also depend on the activity level of the fish. Therefore, fish that are constantly the targets of aggression from tankmates are unable 
to absorb the amount of oxygen sufficient for their metabolism. The fact is that their metabolism becomes more intense because they constantly have to flee, and they lack oxygen even if its content in the aquarium is sufficient for normal needs.
It occurs during overpopulation of the aquarium, at very high temperatures, and as a result of decaying food residues. Many beginners turn off the aquarium aeration at night. And you should never do this. After all, it is at night that the oxygen deficit in the aquarium will be greatest. In the dark, plants do not release oxygen and, just like animals, consume it. Most aquarium suffocations happen at night!
What happens if your fish constantly live with a certain oxygen deficit? Within their capabilities, they will adapt to this. But they will be sick almost constantly. Often you won’t even be able to figure out what exactly served as the source of infection. The thing is, experiencing a constant lack of oxygen, fish cannot properly digest food, grow normally, or possess sufficient immunity to protect their bodies. Therefore, have pity on your fish and provide them with the necessary amount of oxygen.
You can detect a slight oxygen deficit using the same hydrogen peroxide. If adding it to the aquarium in an amount of 15 ml per 100 liters causes a noticeable revival of the fish, causing them to descend to the lower layers of the aquarium where they usually do not swim, this is a sign that you have failed to provide proper water aeration or your aquarium is overpopulated. The oxygen content in the water can be judged by the behavior of the Malayan livebearing sand snail Melanoides tuberculata, which inhabits the bottom of the aquarium. With a sharp decrease in the amount of oxygen in the water, it moves to the aquarium glass or plants.
Causes. • Hypoxia caused by environmental conditions (e.g., insufficient oxygen content in the water). • Due to aquarium overpopulation, the presence of an excessive number of fish, shrimp, snails in the aquarium. From the decomposition of organic matter (food remnants, decaying plants). • Elevated temperature (due to hot climate or thermostat failure) without aeration. • Aeration or water circulation insufficient to provide a high oxygen level for the fish species kept in the aquarium. • As a result of excessive growth of plants and algae, which absorb oxygen and release carbon dioxide at night. • Failure of the aeration system in situations where it is vital to ensure a sufficient amount of oxygen. Such a failure can be caused by equipment breakdown, power outages, or turning off aeration (e.g., at night to reduce noise or save money). • Rapid bacterial proliferation caused by organic pollution of the aquarium (due to poor maintenance or overfeeding of fish). Bacteria consume part of the available oxygen reserves, thereby reducing its amount. • Medications for certain diseases (including formalin and phenoxyethanol) reduce the amount of oxygen dissolved in water. • Excessive oxygen demands due to abnormal metabolic needs – for example, as a result of spawning, participation in fights, or fleeing (from an aggressor or due to fear caused by an external stimulus – for example, if children constantly tap on the glass or run past). • Mouthbrooding cichlid females experience an increased demand for oxygen because they must supply oxygen to the eggs held in their mouths.
• The reverse phenomenon, although rarer but no less harmful, is oxygen supersaturation of the water. It can occur under intense plant illumination by sunlight or 
with intensive aeration. Frequent water changes and mixing warm and cold water can lead to spontaneous release of oxygen and the appearance of tiny air bubbles (white mist), which is very dangerous for fish as gas enters their circulatory system. The first sign of danger is the formation of gas bubbles under the fish’s skin, especially noticeable on the fins. Mass fish mortality results from cardiac embolism.
A high concentration of carbon dioxide in aquarium water affects the ability of fish to deliver oxygen to their tissues and can thus lead to hypoxia. Hypoxia caused by high CO2 levels most commonly occurs during transport and in overpopulated tanks. Carbon dioxide released during fish respiration can reach critical levels (about 25 mg/liter). The critical concentration of carbon dioxide for some particularly sensitive fish may be lower than indicated, and the tolerance threshold may decrease even further if dissolved oxygen concentration is low.
Sufficiently intensive water circulation and surface rippling (achieved through filtration and aeration) will help release carbon dioxide into the atmosphere. Since plants release carbon dioxide in the dark, densely planted aquariums must have sufficiently intensive aeration. Forced CO2 injection to improve plant growth should be used with caution. Test kits for measuring carbon dioxide content in aquarium water are commercially available. There is usually no need to monitor this gas concentration unless it is specifically injected into the aquarium.
Whatever the cause of hypoxia, intensive aquarium aeration is necessary to increase dissolved oxygen levels. A remedy like a partial water change can also be considered if the cause is environmental. In case of extreme emergency, use hydrogen peroxide, which will help rapidly increase oxygen content.
Prevention of diseases caused by hypothermia or overheating.
Hypothermia or overheating occurs when the water temperature gradually drops or rises to values outside the tolerance range for a given species. In tropical and subtropical waters, the temperature practically does not change between night and day. Each fish species is accustomed to its own temperature range in which it feels satisfactory. The natural limits in which certain fish are accustomed to living should not be violated.
Fish are particularly sensitive to sudden, severe water temperature changes, even if they do not exceed the temperature range for the given fish species. Gradual temperature changes rarely pose any threat to the health and life of fish, while sudden fluctuations can cause severe stress, significantly reducing the fish’s resistance to diseases. In general, it can be said that a sharp temperature drop is more likely to cause temperature shock than a sharp increase.
It happens that aquarium water temperature suddenly deviates sharply from the optimum. And what should the optimal temperature be? Most aquarium fish will be satisfied with 22-26°C. Perhaps only Discus and some labyrinth fish prefer warmer water – 28-31°C. Goldfish need cooler water. In aquarium conditions, a temperature range of 18-23°C will suit them quite well. For a fairly long time (a year or more), they can live at 25-27°C, but eventually, they get sick – usually, they develop swim bladder dysfunction.
Heating equipment failure – a burned-out heater in the case of cooling, a failed thermostat in both cases. Cooling can also occur as a result of a power outage, insufficient heater power (this can be critical on cold winter nights when room heating is off), during transport in cold weather. A wide-open window in cold weather can easily chill the room. If there is no heater in the aquarium, the water in it will cool noticeably within a few hours of such ventilation.
It turns out fish can catch a cold too. And then “catch” a bacterial infection or Chilodonella. If there is no thermometer in the aquarium, such temperature fluctuations may go unnoticed by the aquarium owner. Sometimes it is necessary to carefully monitor not only the water temperature but also the air temperature above the water. This is especially important when keeping labyrinth fish that regularly gulp air from the surface. Quite thermophilic Siamese fighting fish (bettas) and dwarf gouramis can fall ill or even die by “breathing” air that is 5 degrees or more colder than the water where these fish live.
Overheating can also be very dangerous. Exceeding 36°C is fatal for most species of aquarium fish, but even 32°C can lead to noticeable trouble. This depends on factors such as nitrate and nitrite content in the water, organic substances, intensity of aquarium aeration, and the degree of satiety of the fish. Overheating is worst tolerated by well-fed fish. Warm water contains much less oxygen, and well-fed fish particularly need it. Since different fish species differ in their tolerable temperature ranges, it is impossible to give the highest critical temperature applicable to all cases. Every aquarist must learn to assess the severity of such an event based on their own experience, depending on the well-being of the fish.
Sometimes aquariums overheat during very hot weather or when direct sunlight hits them. From lamps illuminating the aquarium, if lighting fixtures are selected and positioned incorrectly, especially if the lamps are powerful and the aquarium is covered with a solid lid. The cause may also be the aquarium being located too close to a heat source (e.g., a heating radiator). Overheating can occur during transport when air temperatures are high.
If aquarium fish of any species are kept at 21°C for a long time, they will most likely fall seriously ill or die when placed in water with a temperature of 31°C, even if by mixing water you bring them to this temperature smoothly over 3-4 hours. In any case, raising the water temperature by 4 degrees or more within a few hours poses a serious danger to fish. Severe overheating can cause rapid death.
Too high water temperature also causes disruptions in fish bodily functions. Thus, even with short-term exposure to elevated temperatures, the gill filaments are “scorched”, and an intense whitish mucous coating appears on the skin. The fish’s gill filaments become anemic, covered with mucus, capillaries rupture, and bleeding is observed. After some time, necrotic decay and complete destruction of the gill filaments occur. Saprophytic fungi and bacteria colonize the affected gill areas, a pathological process develops, and mass fish mortality occurs.
In addition, depending on water temperature, pathogens of several associated diseases appear. It should also be remembered that as the temperature rises, the toxicity of most chemical substances to fish and other aquatic organisms increases. When these environmental conditions worsen, the resistance of fish to both excessively high and very low temperatures decreases.
If it is evident that fish are suffering severely (hanging near the water surface or lying on the bottom, suffocating), immediate action must be taken to remedy the situation. A partial water change should be performed, topping up with cold water to quickly lower the temperature to a safer level. If signs of recovery appear in the fish after this, let the temperature drop naturally to normal. Otherwise, consider a further partial water change.
Aeration should be increased to raise dissolved oxygen levels until the water temperature returns to normal. Even with such measures, fish that have suffered severe overheating may die or experience severe consequences (e.g., gill damage, persistent breathing problems, brain damage).
During severe temperature fluctuations, coloration darkens, breathing accelerates, and timidity appears. If the temperature is lower by more than 4 degrees from the optimum for the species, the fish enter a state of shock. Sluggish movements and loss of appetite appear. Slowing or stopping of opercular movements. Fish lie on the bottom, sometimes on their side, or stay in one place, swaying their bodies. If cold exposure lasts too long, heat loss compensation is disrupted. In fish, as cold-blooded animals, body temperature drops as the ambient temperature falls.
An organism’s resistance to cooling depends on its condition (general resistance, body condition, age, etc.). The main cause of death from prolonged hypothermia is considered to be tissue hypoxia and irreversible changes in the nervous system. If the temperature is too high, the oxygen content in the warming water constantly drops. Fish are extremely active, breathe rapidly, and rush up and down the aquarium. They stay near the water surface and try to jump out.
Prevention. Use two independent heaters combined with thermostats in each aquarium, and their total power should be sufficient for the aquarium’s size. If one of them fails to turn off in time, the other will maintain the required temperature. If one of them, conversely, turns on untimely, the other will remain off, and then severe overheating will be less, or at least it will take longer before it occurs.
Do not place the aquarium near heat sources such as heating radiators or fireplaces. Temperature shock can also occur during fish transport if the containers holding the fish lack thermal insulation and are exposed to excessively high or low ambient temperatures. Transferring fish from one condition to another must be done gradually so that the temperature difference does not exceed 3-5°C. During partial water changes, the temperature of the new water must be matched to the aquarium water temperature; when introducing new fish into the aquarium, first equalize the temperature difference.
