Discus (fish)
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[Photo] Common Discus, Symphysodon aequifasciata. Photo by Patrick Farrelly.

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

Taxonomy
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.

Appearance
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, off which the larvae live during their first few days. This behaviour has also been observed for Uaru species.

Diet
In the wild they are opportunistic omnivores and their diet consists of invertebrates and plants. The waters from which discus hail are typically slow-moving, soft and slightly acidic (1 - 5 dGH, pH 4.0 ??? 6.5). Temperature of the water in their natural habitat varies from 25 ??? 30 C ( 82-86 F).

Distribution
The three species of Symphysodon have different geographic distributions. S. aequifasciatus occurs in the Rio Solim??es, Rio Amazonas and the R??o Putumayo-I???? 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. The best cohabitants may be angelfish (although some aquarists claim that keeping them together with angelfish will introduce parasites and/or diseases) and small characides like tetras. Uaru species are also suggested cohabitants for discus. 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.

Many aquarists consider discus to be finicky and not particularly hardy. They often become susceptible to disease and die if not kept in optimal conditions.

Aquarium water chemistry
Aquariums for discus should be kept within a temperature range of 26-31 C ( 82-86 F); a temperature of 29 C (84 F) is thought ideal for adults. Babies and young fish should be maintained at 31 C (86 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.

Captive bred fish adapt very well to harder water and to pH up to 7.2, except when attempting to breed, in which case soft and acidic is best, although it is preferred by the fish anyway. VERY clean water with frequent large volume water changes is necessary for the health of these fish. Never use pure R.O. water or distilled water as some "salts" are necessary.(ie;calcium,magnesium, etc) 100 ppm GH is average. New fish should be quarantined for a minimum of 4-6 weeks in a separate room, separate tank, and separate water changing equipment 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.

Water quality must be very high, as discus do not tolerate pollution of any sort very well. A good tank will be equipped with 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 important, except in the case of a very heavily planted tank with high nitrogen compound grounding capacity and a very small biological load.

Feeding
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 without food before accepting a new type of food. (Therefore, when purchasing discus it is a good idea to ask what they are being fed.) After starving for a month discus will almost always accept a new food, but this may stunt the growth of younger 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 removed.

Beef heart is often fed to discus in order to promote good colouration and quick growth. Pork Heart has also been used to achieve a similar effect. However, concern over the long-term consequences of feeding discus a diet high in mammalian protein has prompted some hobbyists to switch their discus to a diet of krill, a shrimp-like crustacean.

Lighting
Discus prefer low lighting. They are often skittish in the home aquarium, so low lighting together with profuse aquatic vegetation may help them to feel more comfortable in their environment.

Common Colour Varieties
There are three layers of colour on discus: The base colour (which usually ranges from cream to red-brown), the secondary colour (a metallic colour, usually a blue or green colour) 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 golden or reddish base colour. The secondary colour is often striped down the sides of the fish, although many strains (such as 'solid cobalt' or 'blue diamonds') have secondary colour that eventually covers most or all of the fish's body.

There are no rules or authorities on what constitutes a unique colour 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) colour 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.

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