Discus Glossary
Discus keeping has its own vocabulary: dGH, conditioning, true pair, Heckel bar, pigeon trait. None of it is obvious if you are coming from community tanks. This glossary defines the terms you will meet across our variety, care and breeding guides, in plain language and with no assumed background.
Water & Tank
dGH (degrees General Hardness)
A measure of dissolved calcium and magnesium in water. Most successful discus keeping falls between 2 and 10 dGH; wild-caught fish and breeding programs push toward the softer end, while farm-raised stock is far more forgiving. See the water parameters guide.
RO/DI water
Reverse osmosis / deionized water, stripped of minerals and blended with tap water to bring hardness and pH down. Most useful for breeding pairs and very hard tap water; many keepers of farm-raised discus never need it. See the when RO water matters.
pH stability
Keeping pH steady matters more than hitting a specific number. A stable pH that is merely "not perfect" is safer than chasing a target with chemical adjusters and causing swings. See the water parameters guide.
Buffering capacity
Water's resistance to pH change, supplied mostly by carbonate hardness. Very soft water has little buffering capacity and can swing further than you would expect. See the water parameters guide.
Water change
Replacing part of the tank's water with fresh, conditioned, temperature-matched water. It is the foundation of discus husbandry, because it removes the dissolved waste that filtration alone does not. See the water quality monitoring.
Species-only tank
A tank kept for discus alone rather than a mixed community. Discus are held at 82–86°F, well above what most tropical fish tolerate long-term, which is the main reason they get their own setup. See the tank setup guide.
Bare-bottom tank
A tank with no substrate. Common for breeding pairs, quarantine and grow-out, because waste and uneaten food are easy to see and siphon away. See the tank setup guide.
Health
Quarantine
Keeping every new fish in a separate tank for two to four weeks before it meets the main tank, so illness is caught before it reaches existing discus. The single best preventive step. See the common diseases guide.
Hole-in-the-head (HITH)
Hole-in-the-head disease, in which opportunistic flagellate protozoa take hold after chronic poor water or dietary deficiency. Fundamentally a water-quality and nutrition problem rather than primarily a parasite one. See the common diseases guide.
Stunting
Growth that stalls below a fish's genetic potential. Largely irreversible once set, so prevention through generous water changes, space and rich feeding from the juvenile stage is the only real fix. See the common diseases guide.
Darkening
A discus, or a whole group, turning noticeably darker than normal and withdrawing. Usually a symptom rather than a disease, most often tied to shifting water chemistry, an undersized group or aggressive tankmates. See the common diseases guide.
Ich (white spot disease)
A common external parasite that shows as small white spots. It has a free-swimming stage, so the whole tank is treated rather than only visible cases, usually with temperature raised toward 86–88°F to speed its life cycle. See the common diseases guide.
Feeding
Staple pellet
A quality, discus-formulated, high-protein pellet that forms the daily backbone of the diet, supplemented with frozen, live or homemade foods for variety. See the feeding guide.
Beef heart mix
A traditional homemade blend of lean beef heart, vegetable matter, vitamins and a binder, frozen into flat sheets. Very effective for growth and color but demanding to prepare correctly and easy to overfeed. See the feeding guide.
Grow-out
The period when juvenile discus are fed frequent, rich meals and kept in clean water to reach adult size, usually in a dedicated tank. See the breeding guide.
Behavior & Breeding
Pecking order
The social hierarchy within a group of discus. Keeping enough fish together prevents any single individual from becoming the constant target of the dominant ones. See the social dynamics guide.
Conditioning
Preparing a pair for spawning with richer feeding and frequent water changes over a period of weeks, often including live foods such as blackworms or daphnia. See the breeding guide.
True pair
Two discus that bond, claim and clean a shared spot and spawn together, as opposed to two same-sex fish chasing each other. Pairs form most reliably from a group of six to eight juveniles grown out together. See the breeding guide.
Wigglers
Newly hatched discus fry that remain attached to a surface by their yolk sacs, wriggling in place before they become free-swimming and move onto their parents. See the breeding guide.
Body mucus
A nutrient-rich secretion discus parents produce on their flanks, which fry attach to and feed on for roughly their first 10 to 14 days before moving on to finely crushed flake and baby brine shrimp. See the breeding guide.
Line breeding
Breeding closely related fish over successive generations to fix or improve a desired trait, with careful records of which pairs produced which results. See the breeding guide.
Varieties & Origins
Symphysodon
The genus of fish commonly called discus. Taxonomists split the wild populations differently, but the group includes the Heckel discus and the blue, green and brown wild-type complex. See the wild discus.
Wild-caught
Fish collected from their native Amazon habitat rather than bred in captivity. Typically shyer, more demanding of soft water, and slower to settle than farm-raised discus. See the wild discus.
Farm strain
A discus line selectively bred over generations, usually on Southeast Asian farms, for color and pattern. Calmer and more tolerant of average water than wild fish, and the type most hobbyists keep. See the all varieties.
Heckel bar
The noticeably darker, wider third vertical bar through the middle of a Heckel discus's body, the reliable field mark separating it from the other wild discus. See the Heckel discus.
Solid-color strain
A strain bred for an even, saturated single color with minimal patterning, such as Blue Diamond. Judged on evenness and intensity, and quick to show stress. See the Blue Diamond.
Pigeon trait
A selectively bred pattern mutation that suppresses normal barring and pigment density, leaving a pale ground color that breeders use as a base for other color strains. See the Pigeon Blood.
Snakeskin pattern
A fine, netted webbing of lines across the whole body, defined by pattern rather than color and often crossed onto other base strains. See the Snakeskin.
Albino
A recessive genetic trait that removes dark pigment (melanin) and can appear on top of several strains. The reliable tell is a red eye, and the body looks softer and more pastel. See the Albino.