mucous secretion

Discus (fish)

Discus (Symphysodon spp.) are a genus of three species of freshwater cichlid fishes native to the Amazon River basin. Discus are popular as aquarium fish and their aquaculture in several countries in Asia is a major industry.


Discus belong to the genus Symphysodon, which currently includes three species: : The common discus (Symphysodon aequifasciatus), the Heckel discus (Symphysodon discus), and a new species which has been named Symphysodon tarzoo A further investigation published in August 2007 , suggested that the genus held three species: S. aequifasciatus, S. haraldi and S. discus.


Like cichlids from the genus Pterophyllum, all Symphysodon species have a laterally compressed body shape. In contrast to Pterophyllum, however, extended finnage is absent giving Symphysodon a more rounded shape. It is this body shape from which their common name, “discus”, is derived. The sides of the fish are frequently patterned in shades of green, red, brown, and blue. The height and length of the grown fish are both about 20–25 cm (8–10 in).

Reproduction and sexual dimorphism

Another characteristic of Symphysodon species are their care for the larvae. As for most cichlids, brood care is highly developed with both the parents caring for the young. Additionally, adult discus produce a secretion through their skin, which the larvae live off during their first few days. This behaviour has also been observed for Uaru species.


The three species of Symphysodon have different geographic distributions. S. aequifasciatus occurs in the Rio Solimões, Rio Amazonas and the Río Putumayo-Içá in Brazil, Colombia and Peru. In contrast the distribution of S. discus appears to be limited to the lower reaches of the Abacaxis, Rio Negro and Trombetas rivers. S. tarzoo occurs upstream of Manaus in the western Amazon.

In the Aquarium

Discus are shy and generally peaceful aquarium inhabitants. They are sensitive to stress and disturbance or lack of protection. Therefore, it is best to keep discus aquariums in quiet, low traffic areas of a home or office, and to provide the fish with plenty of hiding places. The best cohabitants may be angelfish, although both species are large fishes that require a lot of tank room and some aquarists claim that keeping them together with angelfish will introduce parasites and/or diseases. Small characides like tetras and Uaru species are also suggested cohabitants for discus. Common choices are cardinal and neon tetras as they require similar water parameters. Their schooling behavior also helps to calm the discus as the small fish act as a sort of early warning system if any danger is present. It is noteworthy, however, that small fish may be intimidated or eaten by the discus. Catfish with sucker mouths are less than ideal cohabitants for discus since they sometimes attach themselves on the sides of discus and eat their mucus membranes. Corydoras catfish and loaches are more suitable tankmates as are the true Siamese flying fox a.k.a. algae eaters.

Many aquarists consider discus to be finicky and they are not recommended for inexperienced aquarists. Discus are not particularly hardy and they often become susceptible to stress-borne disease and die if not kept in optimal conditions. Ammonia (produced from fish waste) and suboptimal water temperatures are the biggest threats to the immune system of a discus. In their natural habitat, discus live in essentially distilled water, therefore, the chemicals commonly added to the municipal water supply can be dangerous to the fish. It is for this reason that many discus enthusiasts ultimately choose to do water changes using RO water.

Aquarium water chemistry

Aquariums for discus should be kept within a temperature range of 26-31 C; a temperature of 29 C (84 F) is thought ideal for adults. Babies and young fish should be maintained at 31 C (88 F) degrees. The water should be very soft and slightly acidic; a pH of 5.5 - 6.5 is considered good for wild caught discus.

Most aquarists believe that the water must be changed regularly, but this may not be necessary in a very well planted tank that has adequate biological filtration, lighting and bottom dwelling fish to clean up after the often messy eating discus (in this case "top up" with RO water). A heavily planted tank may also need CO2 injection for the plants to do well. A discus aquarium in order to thrive must always be a discus aquarium first however. Plants and other inhabitants should be chosen with the optimal discus conditions given first priority.

Captive bred fish adapt very well to harder water with a pH up to 6.8 except when attempting to breed, in which case soft and acidic water is best. Actually, maintaining acidic water is always beneficial to these fish because the ammonia which is an ever present danger in contained fish living quarters, is unable to form in a pH under 6.9. At pH 6.9 and lower it begins to convert to ammonium, which is a form more easily tolerated by the fish. A pH in this low (down to about 5.1) range protects from the poison of toxicity while allowing the fish to exist in conditions that more closely mimic the water parameters that they evolved in. However, a pH level below 6.0 will inhibit the growth of beneficial bacteria which live in the biological filter media. While ammonium is far more easily tolerated by fish than ammonia, some biological bacteria is still beneficial even if weekly water changes are performed. While pH levels in the Amazon River Basin range most consistently between a pH of about 5.1 and 6.6, there are many factors at play in nature which are not present in a closed aquarium system that help to break down and metabolize waste. This is why biological filtration is widely considered to be the most important of the 3 types of aquarium filtration (mechanical, chemical and biological).

Planted aquarium or not, it is widely accepted that frequent water changes are the best way to maintain the water quality of a discus tank. In their native environment heavy rains purify the water naturally, and it is probably this constant flushing alongside the acidic "black" water that is this species greatest defense against water-born pathogens and parasites. It is always best to simulate natural conditions when containing captive animals and the discus keeper needs, first and foremost, to address the proper water conditions of their fish. In a home setting the chemical sodium bisulfate, easily and inexpensively acquired from a swimming pool supplies dealer, can be used to lower pH. One time-proven and natural method of lowering pH in the discus aquarium is the use of peat filtration. Aside from the pH-lowering qualities of peat, aquarists have found that the natural tannins and humic acids leached from peat are very beneficial for discus. Using a water holding drum such as a clean, converted, sturdy plastic trash barrel, the fish keeper may experiment with ways of both aging and conditioning the water. Note that when changing the pH of water it is imperative that one wait at least 24 hours until a "bounce back" effect occurs. This is because the pH buffers naturally found in water will fight against the pH change and the aquarium keeper will need to readjust with a further application of the chemical a second time. Though this process may prove demanding and complicated at first, repetition will, as with most things, allow it to become as simple as anything else we do frequently.

In situations where a water holding drum is not practical, such as in an apartment, it is best to try to acclimate discus as closely as possible to the pH of the municipal water supply being used. Most captive bred discus can tolerate pH levels up to 7.0 with no detrimental effects, as long as other water parameters are correct. Large dips and jumps in pH, as can often occur when doing high-volume water changes in tanks with greatly lowered pH levels, can be quite stressful to discus. If the pH level of municipal water is very alkaline then smaller, more frequent water changes (roughly 20% maximum) will be necessary to avoid large and sudden pH fluctuations. Regardless of pH level, when using non-aged tap water it is almost always necessary to use a water conditioner to remove chlorine and other harmful chemical additives.

When attempting to breed discus the water parameters must become even more exacting, and it will usually be necessary for the keeper to process a percentage of the water through an R.O. (reverse osmosis) filter. Never use only pure R.O. water or distilled water as some “salts” are necessary (ie; calcium, magnesium, etc). 100 ppm GH is average. A lack of salts in the water can result in disease outbreaks. Store bought trace mineral additives or the addition of tap water will replace the trace minerals removed through R.O filtration.

Newly acquired fish should be quarantined for a minimum of 4 weeks in a separate tank with separate water changing equipment being used to eliminate the possibility of bringing in an infection to established fish. It is generally accepted that new fish should be added after “lights out” or during normal feeding to lessen stress. As discus, especially wild-caught specimens, are prone to disease brought on by the stress of capture and shipping, a basic knowledge of common discus diseases, (specifically internal parasites and external flukes), their symptoms and the medications commonly used to treat them, can prove invaluable when conditioning new specimens.

Discus do not tolerate pollution of any sort very well. A good tank will [[Aquarium#Design|equipped contain a high capacity biological filter and be fully cycled (which usually takes a month or more.) Ammonia and nitrites should be kept at 0 ppm. Nitrates should also be kept as low as possible. Weekly water changes are needed, except in the case of a very heavily planted tank with high nitrogen compound grounding capacity and a very small biological load. Note however, that a heavily planted aquarium is much harder to keep clean than is a non-planted tank. Many discus keepers have found that a "less-is-more" approach is often beneficial to the health of the fish.

As the natural discus habitat is often devoid of aquatic plants, a basic tank setup can consist simply of fine white gravel substrate and large driftwood pieces. Driftwood is almost essential as it not only provides some shading but also helps keep pH levels acidic and leaches useful tannins into the water. It has been argued by avid discus keepers that fine white gravel similar to that found in the Amazon river bed, helps to aid in digestion and in keeping the gill rakes clean. In contrast, breeding and "grow-out" tanks are often completely bare and contain no substrate and no decorations at all. This aids in tank maintenance where pristine water conditions take top priority.


Feeding discus is sometimes a challenge. They have no unique nutritional requirements; they can be raised on just about any high-protein fish food. However, discus are often extremely cautious about new foods; it is not unusual for them to go for weeks before accepting a new type of food. (Therefore, when purchasing discus it is a good idea to ask what they are being fed and even to ask to see them eat.) After starving for a month discus will almost always accept a new food, but this may stunt the growth of younger fish and cause undue stress to already stressed fish.

It is not advisable to use the starving method for weening discus off of one food for another. Instead, mix the new food with the discus’ preferred food. Over time, the discus will begin to accept the new food, and the old can be discontinued.

Beef heart is often fed to discus in order to promote good coloration and quick growth. Pork and turkey heart has also been used to achieve a similar effect. However, concern over the long-term consequences of feeding discus a diet so high in mammalian protein has prompted some hobbyists to switch their discus to a diet of krill, a shrimp-like crustacean. Discus delight in small live prey, which is the best option for them in the long term. Live black worms, bloodworms, brine shrimp and mosquito larvae are all eagerly eaten by discus. Care must be taken when feeding discus live food though, for bacteria and parasites can be present. For this reason it is advisable to buy live food from aquarium retailers. Live tubifex worms should not be fed to discus AT ALL because it is practically impossible to remove all of the bacteria from them. It is safe however to feed discus freeze dried tubifex cubes as the bacteria and/or parasites have been removed in the process. If live food is unavailable, the best alternative is quality frozen food. The best artificial food for discus is a high quality granulate food. Flakes are also a good option but granules retain vitamins, minerals and other trace elements better than flakes. They always love frozen blood worms. Liquid vitamin supplements and garlic extract are often added to frozen and live foods. Simply adding the recommended dose (usually 1 or 2 drops) to a portion and allowing it to soak in over a period of a few minutes is sufficient.


It is a myth that discus prefer low lighting. They are often pictured in dark aquariums as this is the best way to show their colors, however discus do not require any special lighting. They will quite happily live in a well lit tank just much as most other tropical freshwater fish. They do however, appreciate some shaded areas in the aquarium.

Common Color Varieties

There are three layers of color on discus: The base color (which usually ranges from cream to red-brown), the secondary color (a metallic color, usually a blue or green color) and the black pigment that makes up the black vertical bars and allows the fish to darken and lighten at will.

Most discus strains have either a yellow or reddish base color. The secondary color is often striped down the sides of the fish, although many strains (such as ‘solid cobalt’ or ‘blue diamonds’) have secondary color that eventually covers most or all of the fish’s body.

There are no rules or authorities on what constitutes a unique color variety or what to call it. A particular form may or may not breed ‘true’ (with offspring very closely resembling the patterns of their parents.) Generally all of the common, established forms breed true . The exact patterning of the secondary (blue/green) color is like a fingerprint; it develops chemically rather than being set precisely by genetics . The offspring of two ‘spotted’ discus will likely have spots, but not in the exact same size/position as their parents.

Notable color varieties:

  • Brown: The most common color form in the wild; these fish have a brownish base color with minimal stripes of secondary color only along the head and fins.
  • Blue/Green: Similar to the Brown, but with more secondary color (either bluish or greenish.)
  • Royal Blue: The secondary color forms stripes across the entire body, with a golden base color. These splendid fish are the basis of many of the developed color strains, and are primarily responsible for the early fame of discus. Royal Blues can usually be readily distinguished from selectively bred color forms by their less even base color, with the golden color becoming a brighter yellow around the breast area.
  • Red Spotted Green: A reddish base color with greenish secondary color with ‘holes’ in it (producing spots of the red base color showing through.) This handsome color form is extremely rare in the wild, but is produced by several breeders.
  • Heckel: Possibly a separate species, Heckels are identifiable by two vertical black bars that are much thicker than the others.

Common Bred forms:

  • Red Turquoise: A red-brown base color with stripes of blue-green secondary color, normal black pigmentation (bars).
  • Solid Cobalt: Golden or light brown base color, but when fully mature covered with a blue secondary color. Black pigmentation may be normal or incomplete (some vertical bars missing.)
  • Blue Diamond: Essentially a ‘solid cobalt’, but the black bars have been completely removed through selective breeding. The reduction in black pigment gives these fish a bright, lighter blue color than most ‘solid’ discus.
  • The Pigeon Blood mutants: These fish have a gene that disrupts the distribution of the black pigment. As a result, they lack vertical black bars (but often have ‘pepper’). The lack of black pigment makes their base color much lighter and brighter; as a result, discus with this mutation may show brilliant red or yellow (or even pale cream) primary color. Most of these strains are no longer called ‘pigeon bloods’ per se, but are easily identifiable by the bright base colour, pepper, and lack of black vertical bars. All pigeon bloods are the descendant of a single fish found in Eastern Asia in the 1980s. Since the trait is dominant and appears to be controlled by a single gene, fish bearing this mutation can be crossed with any other color strain to produce novel new ‘pigeon blood’ types. Pigeon bloods do have one drawback: They cannot darken at will (as normal discus can). This can make it difficult for them to raise fry, which are attracted to their parents by seeking out a dark object. (Normal discus darken when spawning or stressed.) High quality pigeon blood types have few or no ‘pepper’.
  • Snake-skins: These fish have a mutation that makes their patterning ‘tighter’; as a result, they have about twice as many black vertical bars, but also have tighter, finer secondary color patterns than normal discus.

Breeding discus fish

A bare bottom twenty or twenty-seven gallon tall tank is ideal for breeding discus. Discus lay their eggs in the same manner as angels, and so a vertical surface should be provided for them to deposit their eggs on. An inverted flowerpot is sometimes used. A potted plant or two can be added to the breeding tank if so desired, and will provide some shelter for the pair, but this is not essential. As far as filtration goes, a sponge filter should be used to handle the biological load along with an outside power filter to pick up any debris that may be in the water.

Discus come from the warm, soft, acidic waters of the Amazon River and thrive when these conditions are replicated in the home aquarium. For both general maintenance and breeding of discus the pH should be kept at 6.5 and the temperature around 86 F. Any alterations that need to be made to the water chemistry should always be done prior to the water being added to the tank. For general maintenance water changes should be performed weekly. However in the breeding tank, a small water change should be done every day, or every second day. Frequent water changes increase appetite and promote mating activity in discus. It’s no coincidence that discus will often spawn after a water change.

If good water quality is maintained the discus will have large appetites and should greedily accept any food offered to them. Spawning discus should be fed frozen blood worms, beef heart, Tetra Color Bits, frozen or live brine shrimp, or live white worms. If feeding beef heart, one must be careful that none is left over because it will foul the water very quickly. Live tubifex or black worms should never be fed to discus at any time, as they will introduce parasites to the tank. A breeding pair will lay eggs as often as every week, as many as fifteen times. They will usually go through two of these spawning cycles in a year. The eggs take 48 hours to hatch, and are free-swimming another 72 hours later. Immediately upon becoming free-swimming the fry will move to their parents’ sides, and start feeding off the mucous secretion that are produced by the parents during this time. The fry will feed off their parents’ sides for as long as you leave them together, but they should be offered newly hatched brine shrimp after being free-swimming for five days.

The fry should be removed between two and three weeks after reaching the free-swimming stage, as leaving them with the parents any longer may be hazardous to the parents’ health. The youngsters will actually reach a point where they can start ripping off scales and bits of flesh from the parents. Once the fry are removed the pair will spawn again in short order. The fry, now in a tank of their own, should be fed six or more times a day. The best foods to give the fry are newly hatched brine shrimp and chopped blood worms. For the first few weeks there is noticeable daily growth. In the fry tank it is important to do a partial water change every night after the last feeding.


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