Most common diseases of discus fish.
Almost all *Symphysodon* inhabit clean flowing or slightly flowing waters where dissolved organic matter is practically absent. Therefore, one of the main conditions for keeping these fish in an aquarium is the constant maintenance of cleanliness and partial water changes. Nitrites and nitrates resulting from the decomposition of fish excrement, food residues, vegetation, etc., pose a great danger to discus fish. With an increased content of nitrogenous substances, the fish change color (the body becomes almost black), they stop taking food, and their growth slows down. Later, whitish-bluish mucous spots appear on the body surface – it seems as if it is covered with a film. The eyes become cloudy and are also covered with a white film.
In some cases, with an increase in organic matter in the aquarium water, the fish contract so-called fin rot. A white border appears on the fins, more often on the caudal fin, and then the soft tissue of the fins disintegrates (necrosis). Damaged blood vessels begin to bleed. As a result of this disease, the fins of discus fish are significantly shortened, but subsequently, the fin tissue partially regenerates. It is interesting to note that other fish kept in the same aquarium with discus do not exhibit this disease. When the fish darken in color or a film-like coating appears on their body, it is necessary to urgently replace ½-1/3 of the water (settled water is added) and thoroughly clean the bottom and substrate of the aquarium.
It is believed that when treating fish, water replacement should be carried out based on the following calculation: when keeping two adult discus in a 200-liter aquarium, 10 liters of water should be replaced every two days. If the film on the eyes and body surface does not disappear, the fish should be kept for three minutes in a 2% solution of sea salt or table salt. It is good to alternate salt baths every other day with baths of a pale pink potassium permanganate solution. This procedure should be carried out until the signs of fin rot disappear. When conducting therapeutic or preventive baths, you should carefully monitor the water temperature: the difference with the aquarium water should not be more than 1.5°C, otherwise, the fish may experience temperature stress.
Intestinal diseases of discus caused by improper feeding give aquarists a lot of trouble. Many enthusiasts feed discus with tubifex worms (*Tubifex*). This is a good food, but we must not forget that they are harvested in water bodies heavily polluted with domestic sewage and organic matter. Tubifex feed on the bacterial flora of water bodies and carry in their intestines a large number of opportunistic pathogenic bacteria from the *Aeromonas* group, which on weakened fish turn into a group of dangerous (pathogenic) organisms and are capable of causing infectious dropsy or scale protrusion not only in discus but also in other fish.
To avoid this disease, purchased tubifex should be kept in a cool place for 5-6 days, washing them morning and evening. During this time, tubifex releases a large amount of bacteria contained in their intestines into the water. On the sixth day, the worms usually acquire a pinkish color and practically stop releasing a black silt-like mass. Now they can be fed to the fish.
When feeding discus with *Artemia salina* and decapsulated crustacean eggs, intestinal enteritis and liver damage are often observed in juveniles. Trout-farming laboratory staff found that this disease is associated with the presence of a large number of *Salmonella* bacteria in brine shrimp eggs, the toxin of which causes enteritis and fish death. To avoid this phenomenon, *Artemia salina* eggs must be kept dry at a temperature of 2-5°C or in salt solutions (up to 20%) in refrigerators at the same temperature.
In older discus, digestive system disorders are noted when feeding with “Ocean” paste and foods containing shrimp additives. The fact is that if stored open, they quickly oxidize and become toxic. Therefore, they must be kept in the freezer in tightly closed jars or tied plastic bags. Usually, discus suffering from enteritis due to improper feeding exhibit darkened coloration, abdominal distension, reddening of the anus, and discharge of bloody clots.
In this case, you need to place the discus in a 1.5% table salt solution for 10-15 minutes, then transfer them to a clean aquarium and gradually raise the water temperature in it to 30-32°C, combined with strong aeration. For three days, the fish should be kept on a fasting diet. During this time, due to the action of salt, high temperature, and oxygen dissolved in the water, the metabolism of the fish increases, and toxic substances are intensively excreted. On the fourth day, the discus are given fresh, complete food — bloodworms (*Chironomus*), *Corethra*, large *Daphnia*, lean meat, etc.
If discus are kept with other aquarium fish without frequent water changes, with periodic temperature fluctuations within 5-6°C, or at a water temperature no higher than 26°C, they contract tuberculosis. The disease is chronic. It is accompanied by stunted growth, scale loss, exophthalmos (pop-eye), and the appearance of whitish ulcers on the body from which a white “worm” seems to be released. In the internal organs of the fish, especially in the liver and kidneys, a large number of brownish-gray pseudocysts containing bacteria are observed. The fish lose weight sharply, and their abdominal region becomes severely drawn in. Sick fish cannot be cured, so they are destroyed, and the aquarium is thoroughly washed and disinfected with ozone.
Severe contamination of the aquarium can cause a chronic fungal disease in discus (ichtyosporidiosis, ichthyophoniasis). Fish can also get sick when fed meat and caviar of marine fish, as well as when kept in community aquariums where other fish may be carriers of ichthyosporidiosis. The signs of the disease are very diverse: the abdomen enlarges, exophthalmos appears (often unilateral), internal tumor-like growths, ulcers on the body surface, soft fin tissues fall off without visible foci of inflammation. The fish begin to periodically rotate and swim in circles.
A definitive diagnosis can only be made after autopsy of the fish and the discovery of cysts with the fungus *Ichthyosporidium* in various organs. There are currently no control measures for this disease. Usually, sick fish are destroyed or attempts are made to obtain fry from them and separate them from the parents even before they switch to active feeding. Such fry are raised artificially.
Parasitic diseases can also be introduced into the aquarium with discus (along with the fish themselves and food). Oodiniasis causes significant damage to discus. The insidiousness of this disease lies in the fact that the fish actively takes food until the last day, but one day, usually in the morning, we find it dead. At the same time, mass mortality of fish does not occur; only individual specimens die. With oodiniasis, a faint yellowish coating appears on the body surface and fins. The fish looks as if it is slightly dusted with flour. Scrapings from the body surface under 10x magnification reveal a large number of grey, oval-pear-shaped bodies.
To combat the disease, the use of the dye basic violet K yields good results. The stock solution is prepared at the rate of 0.5 g of dye per 1 liter of water. Then the prepared solution is added to the aquarium (10 ml per 100 liters of water). Three days later, the treatment is repeated. The stock solution can only be used within 24 hours as it quickly inactivates. Molluscs must be removed from the aquariums, while plants can be left. On the 4th-5th day, the preparation breaks down, and the water clears.
Discus can also be infected with costiosis via food. In this case, a bluish coating and severe redness appear on the body surface of the fish, and the gills are covered with mucus. Usually, a fish sick with costiosis swallows air from the aquarium surface even with good aeration, frequently jumps out of the water, rushes about, and rubs against hard underwater objects and plants.
Discus also suffer from chilodonelliasis, the external signs of which are the appearance of heavily slimed areas on the body and clouding of fin coloration. The fish refuse food and die. And another discus disease widely common among aquarium fish is ichthyophthiriasis. The pathogen is usually introduced into the aquarium with fish or poorly washed *Cyclops*, bloodworms, tubifex, *Corethra* caught in water bodies where wild fish reside. Greyish specks appear on the body surface and fins; it seems as if the fish is sprinkled with semolina.
Among internal parasitic protozoa, the causative agents of hexamitiasis — *Spironucleus sp.* and *Hexamita sp.* — cause great damage to discus. Juveniles and middle-aged fish are most susceptible to this disease. Older individuals are often only carriers of hexamits, but with worsening keeping conditions and poor nutrition, they also get sick and die. The hexamitiasis pathogen usually affects the intestines and bile ducts of the discus liver. Signs of the disease: darkening of the fish coloration, refusal of food accompanied by periodic starvation for 4-6 days, destruction of soft tissues and fins, a sharp slowdown in growth, and the appearance of reddish fecal masses or transparent tubes from the anus. The fish lose weight and become timid. When infected with hexamits, they usually do not reach their natural size and do not participate in spawning. The disease is definitively diagnosed by examining scrapings from the walls of the affected intestine and identifying motile, small, oval-shaped parasites with three pairs of flagella at the front of the body in smears and scrapings.
To treat this disease, furazolidone is used at the rate of 0.02 mg per 2.5-3 cm long fish. The drug is administered with food once a day. Good results are obtained by using ossersol and povarinol at a dose of 0.01 mg per 3 cm long fish (for three days). Treatment of fish should be carried out outside the aquarium. The aquarium where *Hexamita* was detected must be emptied of water and substrate, and the bottom and walls disinfected with a 2% NaOH or KOH solution. After disinfection, it should be thoroughly rinsed and then wiped dry with a cloth. After standing dry for an hour (parasite cysts cannot withstand drying), the vessel can be filled with water. Aquatic plants should be removed from the substrate and rinsed under running water.
Monogenean trematodes — worms that affect the gills and body surface of fish — cause great damage to discus of various ages. They are invisible to the naked eye as their sizes reach up to 300 µm. The worms lay eggs from which motile larvae emerge and settle on the gills of discus. Infected fish scratch against the substrate and plants, shake their fins, and frequently twitch their entire body. They stay near the water surface and gulp air with their mouths. Discus fry 1.5 cm long usually die if 38-50 worms parasitize their gills. For 0.5 cm long fry, 12-14 worms are enough. Many aquarists, having obtained offspring from discus, cannot protect them from these parasites in the first 10-18 days. Monogeneans are highly specific parasites unique to this particular species of fish. Only those parasitizing their gills are dangerous to discus. They do not infect other fish.
To successfully combat these parasites, 0.10-0.15% chlorophos baths should be used (once for 20 minutes). In the aquarium where the infected fish were, water and substrate do not need to be changed, but the discus must be removed and the temperature maintained at 26-28°C for ten days. During this time, larvae hatch from the parasite eggs resting at the bottom of the aquarium, which, failing to find a host, soon die. If the aquarist cannot wait so long, the water and substrate in the aquarium must be changed, plants washed, and the walls and bottom dried and wiped with a cloth. After this, the aquarium can be restocked with fish. Good results are obtained by using ammonia baths (1 ml of 25% ammonia solution per 1 liter of water). The fish are kept in it for 1-1.5 minutes, constantly monitoring them. If they begin to thrash or lie on their side, they are removed from the bath. Due to the fact that discus are extremely timid and easily injured during capture, it is advisable to carry out treatment in a specially made gauze or gas net lowered into the bath; at the same time, care must be taken to ensure the fish do not jump onto the floor. Just in case, a wet soft terry towel should always be ready. It is used to cover a fish that has jumped out, preventing it from thrashing, and it is washed with a weak potassium permanganate solution.
Very often, fish diseases are caused by roundworms (nematodes) of the genus *Capillaria*, which parasitize the intestine. The worms are thin, 1-2 cm long. They are also found in other animals. The disease in fish can be diagnosed by examining feces under a microscope (at high magnification) or by finding oval-shaped eggs with a “plug” at the end in the feces. Discus infected with capillaria lose weight, lose appetite and mobility, and their abdomen swells. Feces are often white. The disease is especially severe if the fish is also infected with hexamitiasis. Good results are obtained by using ossersol at the rate of 0.01 mg per 12-15 cm long discus; administration is twice every other day along with food.
Quite often, whitish crater-like ulcers appear on the body and head surface of discus, from which cheesy “worms” emerge. The cause of this disease is unknown, and control methods have not been developed. However, if the aquarium is cleaned and part of the water is constantly changed, the disease disappears.
Shell-Dwellers cichlids of Lake Tanganyika.
Discus Care. Feeding, Breeding Discus. 
