Understanding Discus: Origin and Biology
Discus (Symphysodon genus) are native to the warm, soft, acidic blackwater tributaries of the Amazon River in Brazil. The Rio Negro and its tributaries run at pH 4.5-6.0, temperatures of 28-32°C, and extremely low mineral content (TDS under 50 ppm, hardness near zero). These are not average freshwater conditions — they are among the most chemically extreme environments in which aquarium fish are kept.
This origin explains every challenge discus present in captivity. They are sensitive to water quality deterioration, stressed by pH fluctuation, susceptible to diseases that other cichlids brush off, and slow to reproduce when conditions are suboptimal. However, discus available in the hobby today are largely tank-bred — many generations removed from wild genetics — and most commercial strains are noticeably more adaptable than wild-caught fish.
Three recognized species exist: Symphysodon aequifasciatus (common discus), S. discus (Heckel discus, with a distinctive bold stripe), and S. tarzoo (Tarzoo/Green discus). Commercial aquarium strains span dozens of color morphs: Red Marlboro, Blue Diamond, Leopard Snake Skin, Pigeon Blood, and many more.
Water Parameters: The Non-Negotiables
pH: 5.5-6.8 for captive-bred discus. Wild-caught require 4.5-6.0. The closer you can stay to the lower end of this range, the fewer bacterial and parasitic diseases you will encounter. Measure pH daily when starting out — it drifts as CO2 is consumed during the day and accumulates at night.
Temperature: 28-31°C (82-88°F). Discus are among the highest temperature freshwater fish in the hobby. Higher temperatures accelerate their metabolism (requiring more feeding) but also suppress many pathogens, including Hexamita (hole-in-head disease). Do not drop below 28°C for extended periods — temperature crashes cause immediate immune suppression.
TDS (Total Dissolved Solids): below 150 ppm for captive-bred, below 80 ppm for wild-caught or breeding. TDS represents all dissolved minerals in the water. Phnom Penh tap water typically runs 200-400 ppm TDS — too high for discus without treatment. A reverse osmosis (RO) filter produces near-zero TDS water that you then reconstitute to the correct mineral level using targeted remineralizers.
Ammonia and nitrite must be zero at all times. Discus show immediate stress responses (darkening, hiding, clamped fins) at ammonia levels that most fish tolerate. Nitrate should stay below 10 ppm — this requires large, frequent water changes.
Water Change Protocol for Discus
Discus keeping is fundamentally a water change discipline. Professional discus breeders in Southeast Asia typically perform 50-80% water changes daily. Home hobbyists typically do 30-50% every other day as a minimum. The frequency depends on fish load and tank size — more fish in less water demands more frequent changes.
The water you add must match the tank exactly in temperature and be dechlorinated. Adding water that is even 2-3°C cooler than the tank triggers stress responses and disease outbreaks within 24-48 hours. Pre-heat replacement water in a bucket with an immersion heater, verify temperature with a thermometer, then slowly siphon it in.
For RO water users: always add your mineral mix to the RO water BEFORE adding it to the tank. Pure RO water has zero buffering capacity — adding it directly to a tank can cause sudden pH drops that stress or kill fish.
- ✦Use a timer-controlled automatic water changer if doing daily changes — reduces manual labor dramatically
- ✦Always keep a thermometer in your water change bucket, not just the tank
- ✦Add a dechlorinator rated for heavy metals if your tap water contains copper (check with local water authority)
- ✦Test pH in both your source water and tank before each change — the difference should be under 0.2 pH units
Feeding Discus: Nutrition and Schedule
Adult discus should be fed 3-5 times daily, young fish (under 6 months) up to 8-10 times daily. Discus are relatively slow feeders that require undisturbed feeding periods. Any stress during feeding — a person walking past, lights switching, loud noise — causes them to abandon their meal, leading to nutritional deficiencies over time.
The beef heart paste diet has been the cornerstone of discus nutrition for decades. A basic recipe: 500g lean beef heart (all fat removed), 2 fresh prawns, 2 hard-boiled egg yolks, 1 teaspoon spirulina powder, 1 teaspoon vitamin supplement. Blend until smooth, pour thin layers onto plastic sheets, freeze, then break into feeding-sized pieces. Feed as primary food 3-4 times daily.
Supplement beef heart with high-protein pellets (Sera Discus Granules, Tetra Discus), frozen bloodworms, and occasionally fresh live blackworms. Bloodworms from Cambodia can carry parasites — freeze at -18°C for 48 hours before feeding to kill parasites, or buy from a reputable supplier.
Discus Diseases and Treatment
Hexamita/Spironucleus (Hole-in-Head Disease) is the most feared discus disease. It presents as small pits or craters on the head and lateral line, accompanied by stringy white feces, appetite loss, and rapid weight loss. It is caused by internal flagellate parasites that proliferate when fish are nutritionally deficient or immunocompromised. Treatment: Metronidazole (Flagyl) at 250mg per 40 liters, dosed every 24 hours for 10-14 days with water changes before each dose. Improve nutrition simultaneously.
Black disease (extreme darkening) is not a disease but a distress signal. Discus darken dramatically when stressed by poor water, aggression, disease, or sudden environmental changes. Identify and resolve the stressor — do not medicate for darkening alone.
External parasites (Chilodonella, Costia, Trichodina) cause mucus coat disruption, white patches, and scratching. Treat with formalin-malachite green combination (Rid-ich Plus or equivalent) at half the standard dose for discus — they are sensitive to many medications. Always start at half dose and observe before completing a full treatment.