Fish
Discus
Symphysodon · also called discus fish
A discus is a cichlid pressed into a coin. That round, flat body lets it drift unseen among the drowned roots of the flooded Amazon forest, and it has made the fish the most coveted prize in the freshwater hobby. Its strangest feature is hidden: for the first weeks of life, discus fry eat nothing but a special mucus that both parents grow over their own skin.
Overview
The discus, Symphysodon, is a round, disc-shaped cichlid of the calmest backwaters of the central Amazon — flooded forest, still bays, oxbow lakes — in warm, soft, acidic, tea-coloured water. It feeds on invertebrates, larvae, and worms among the leaf litter. Wild fish come in brown, blue, green, and barred Heckel forms; Asian breeders have produced dozens of vivid aquarium strains.
Feeding on a parent
After hatching, discus fry cannot feed themselves, so both parents secrete a thick, protein-rich mucus over their flanks and the young graze on it, switching from one parent to the other for two to three weeks — a fish equivalent of nursing.
The demanding king
Long called the “king of the aquarium”, the discus needs warm (28–30°C), very clean, soft water, frequent large water changes, a calm tank, and a group of six or more — it sulks and gets bullied if kept alone or as a single pair. Adults reach a saucer-sized 15–20 cm and live around 10 years; stunting from poor water while young is permanent.
Status
Least Concern — wild discus remain widespread across the central Amazon, and the trade is now almost entirely farm-bred. Dams on Amazon tributaries, deforestation, and mining pollution are the long-term threats.
Discus: common questions
Why are discus considered hard to keep?
Discus ask for more than most tropical fish on several fronts at once. They need warm water, around 28 to 30 degrees Celsius, which is too hot for many common tankmates. They need very clean water with almost no nitrate, which for a big, deep-bodied, messy fish means large, frequent water changes, often several times a week for growing fish. They prefer soft, acidic water. They are shy and easily bullied, so they need a calm tank and a group of six or more. And they are slow, deliberate feeders that can be outcompeted. None of this is difficult on its own, but together it makes discus a fish for someone ready to commit to a routine.
Do discus really feed their babies with their skin?
Yes, and it is one of the most remarkable behaviours in fish. After the eggs hatch, the fry spend a day or two attached to the spawning site, then swim to the parents and begin nibbling at their sides. Both the male and the female produce a thick layer of mucus rich in protein, fat, and antibodies specifically to feed the young, and the fry graze on it constantly, moving between the two parents, which take turns. This lasts about two to three weeks, until the fry can take other food. A few other fish do something similar, but in discus it is the sole food for the young at first.
How big do discus get and how long do they live?
A healthy adult discus reaches about 15 to 20 cm across, roughly the size of a saucer, and a well-kept discus lives around 10 years, sometimes more. They grow fastest in the first year, and stunted growth from poor water quality or an unstable environment while young is common and permanent, producing a small, oddly proportioned adult. Because of their eventual size and their need to be in a group, a proper discus tank is large, at least 250 litres for a group, and taller than average to suit the body shape.
What is the difference between a discus and an angelfish?
Both are cichlids from the same slow Amazon waters and both are tall, laterally flattened, and pair-spawning, but they differ in shape, size, and demands. An angelfish is triangular, with long trailing fins and a pointed profile, grows to about 15 cm tall, and is a semi-aggressive ambush predator that is fairly easy to keep. A discus is round, with short fins and a smooth disc outline, grows a little larger, is peaceful to the point of being timid, and needs warmer, cleaner, softer water. Angelfish are a good intermediate fish; discus are an advanced one. They are sometimes kept together, since their water preferences overlap, though the angelfish tends to outcompete the discus at feeding time.