Darkness — the Universal Early Warning Sign
The most important disease monitoring skill for discus keepers is learning to read colour. A healthy discus is vibrantly coloured with erect fins and active swimming behaviour. A discus in early distress darkens — its base colour deepens from bright to dark, its metallic highlights fade, and it may develop dark vertical stress bars across its body. This colour change is the fish's equivalent of a fever — a non-specific signal that something is wrong, triggering before any other symptoms appear. Learning to distinguish normal resting colour from stress darkness is skill number one.
Darkness in discus can indicate almost any problem: poor water quality, temperature drop, new fish stress, disease onset, internal parasites, social bullying, or imminent spawning behaviour. Your response to darkness should always start with water parameter testing — check temperature, pH, ammonia, and TDS first. If parameters are in range, observe which fish are dark (individual or group) and whether they are eating. A single dark, non-eating fish with clamped fins points toward infection or injury; a whole tank of dark fish usually indicates a water quality event or temperature crash.
Consistent monitoring — observing your fish at the same times each day — builds the baseline knowledge needed to identify abnormal darkness quickly. Many experienced Cambodia discus keepers check their tanks first thing each morning before feeding, specifically looking for any fish darker than normal. This morning check is your earliest disease warning system. Catching a problem on day 1 of symptoms versus day 4-5 dramatically changes the treatment difficulty and outcome. Document unusual behaviour in a simple log so you can identify patterns over time.
- ✦Photograph your healthy discus in good lighting to create a colour baseline — compare against it when you suspect stress
- ✦Darkness in all fish simultaneously after a water change = water parameter problem, check immediately
- ✦A single dark fish hiding behind driftwood = likely bullied or sick, isolate to quarantine tank for observation
Hexamita and Hole-in-the-Head Disease — the Primary Discus Killer
Hexamita (Hexamita sp.) is an intestinal flagellate protozoan and the most devastating disease affecting discus worldwide. In early stages, Hexamita manifests as white stringy feces (vs normal dark short feces), loss of appetite, darkening, and lethargy. In advanced stages, it causes erosion of the lateral line system and head pits — a condition called Hole-in-the-Head (HITH) that leaves permanent disfiguring craters on the fish's face and lateral line even after successful treatment. Prevention is always superior to treatment for this disease.
Hexamita thrives under specific conditions: poor water quality (high nitrate, low oxygen), temperature below optimal (below 28°C), chronic stress, and overcrowding. This means that a discus kept in optimal conditions rarely develops clinical Hexamita — most fish carry the protozoan at subclinical levels that only become problematic when husbandry deteriorates. The first-line prevention strategy is therefore maintaining pristine water quality, correct temperature, adequate space, and low stress. Medication should be the last resort, not the first response.
Treatment for Hexamita uses Metronidazole (Flagyl) — a prescription antiprotozoal antibiotic available from veterinary suppliers. The standard dosage is 250-500 mg per 40 litres of water, administered in the quarantine tank with a 25% water change every 24 hours and redosing. Treatment duration is 5-7 days minimum. In Cambodia, Metronidazole tablets are available from pharmacies (it is also used in human medicine for similar parasitic infections) and from veterinary suppliers in Phnom Penh. Always treat in the quarantine tank — medication in the main display tank destroys beneficial bacteria.
- ✦Keep Metronidazole tablets in your discus medicine cabinet at all times — Hexamita progresses fast and waiting to source medication costs days
- ✦Feed Metronidazole mixed into food (crush tablet, mix into beef heart paste) for better bioavailability than water treatment alone
- ✦Raise temperature to 32-33°C during Hexamita treatment — the elevated temperature accelerates the protozoan life cycle and increases Metronidazole effectiveness
Gill Flukes — Rapid Breathing and Hidden Damage
Gill flukes (Dactylogyrus sp. and Gyrodactylus sp.) are microscopic flatworm parasites that attach to gill tissue, causing physical damage that manifests as rapid or laboured breathing, flared gill covers, abnormal surface behaviour (gasping), and reduced feeding. Infested fish look normal from a distance until you observe their breathing rate. A healthy discus breathes at 60-80 beats per minute at rest; gill fluke infested fish breathe at 100-120+ beats per minute continuously. Count respiration rate if you suspect gill issues.
Gill flukes are introduced primarily through new fish that have not been adequately quarantined. Even apparently healthy fish from reputable suppliers can carry low fluke loads that become problematic under stress in a new environment. This is the primary reason quarantine is mandatory for all discus — a fluke-infested new fish in a quarantine tank is treatable; the same fish added directly to a display tank with 10 healthy residents creates a tank-wide outbreak requiring full treatment of the main display, which is far more disruptive.
Treatment for gill flukes uses Praziquantel (common brand: PraziPro) at 2.5 mg per litre of water. Praziquantel is effective, fish-safe, and does not destroy biological filtration at therapeutic doses. Treat for 5-7 days with daily 25% water changes and redosing. A second treatment course 10 days after the first catches any flukes that hatched from eggs during the first treatment. Praziquantel is available from aquarium health product suppliers in Phnom Penh and through online marketplace imports from Thailand.
- ✦Include Praziquantel treatment as a routine prophylactic for ALL new discus during the first week of quarantine — treat before symptoms appear
- ✦Observe gill covers from the front of the tank — one flared gill cover consistently is a sign of unilateral gill infection or irritation
- ✦Salt (1-2 tablespoons non-iodised per 10L) as a stress reducer during fluke treatment improves fish survival but is NOT a fluke treatment on its own
Internal Parasites — White Feces and Wasting
Internal parasites are common in discus, particularly capillaria (threadworms), camallanus (red worms protruding from anus in severe cases), and various tapeworm species. The primary visible symptom is white or pale stringy feces that hang from the fish rather than being expelled normally. Secondary symptoms include progressive weight loss despite eating (the "wasting" syndrome), a pinched-in appearance behind the head, sunken eyes in severe cases, and lethargy. Wild-caught discus and fish from unclean suppliers are at highest risk.
Levamisole hydrochloride is the treatment of choice for most internal nematode parasites in discus. Dose at 1g per 100 litres of water in a treated tank, fast the fish for 24 hours before treatment to maximise drug uptake, and maintain elevated temperature of 30-32°C during treatment. A single dose is often effective; repeat in 10 days to address any newly hatched larvae. Levamisole is available from veterinary suppliers and some pharmacy distributors in Cambodia. For tapeworms, Praziquantel is the preferred treatment — some keepers do a combined Levamisole + Praziquantel treatment to address multiple parasite types simultaneously.
Prevention of internal parasites centres on food hygiene and quarantine. Tubifex worms are the primary vector — never feeding them eliminates the largest single risk. Beef heart should be sourced from food-grade butchers and frozen before use — freezing at -18°C for 48 hours kills most parasite stages. New fish through full quarantine protocol as described in the tank setup article is the other essential prevention measure. Internal parasites do not cross-infect from plants, rocks, or equipment — they are introduced only through fish and infected food.
- ✦Freeze all frozen foods for 48 hours before first feeding — home freezers at -18°C kill most parasites including capillaria eggs
- ✦Examine feces under a magnifying glass if you suspect parasites — white stringy feces are diagnostic even without microscopy
- ✦A discus that eats well but loses weight progressively almost certainly has internal parasites — treat promptly before the fish is too weakened to recover
Discus Plague — Prevention Is the Only Option
Discus plague (also called discus epidemic or "the shimmies") is a collective term for a poorly understood viral syndrome that causes mass mortality in discus populations. Affected fish display shimmying (rapid side-to-side vibration), extreme darkness, clamped fins, refusal to eat, and rapid death within 3-7 days of symptom onset. There is no reliable cure once a tank is affected — the primary viral agent has not been fully characterised and no antiviral exists for fish. Prevention through biosecurity is the only effective strategy.
The primary entry route for discus plague is the introduction of infected fish. Asymptomatic carrier fish from apparently healthy tanks can introduce the pathogen into a new environment where stress triggers an outbreak. The best prevention is strict quarantine (4 weeks minimum), sourcing fish only from reputable suppliers with demonstrably healthy colonies, and never introducing fish from sales or shows directly to your main tank. In Cambodia, where discus are sometimes sold through informal channels without proper quarantine facilities, this risk is elevated.
If discus plague does appear in your tank, the established response protocol is: raise temperature to 33-34°C (heat therapy reduces viral replication rate), add salt at 3 tablespoons per 10L, increase aeration dramatically (high temperature reduces dissolved oxygen), perform 50% water changes daily, and add Metronidazole to address secondary bacterial infection. Some fish will survive; those that do develop partial immunity. The survivors' tank, equipment, and substrate must be considered contaminated and sterilised with bleach before use with new fish. Discus plague is the most feared disease among serious discus keepers globally.
- ✦Never buy discus from a tank where even one fish looks slightly dark or shows clamped fins — the whole tank is suspect
- ✦Elevate quarantine temperature to 32°C for the first 2 weeks of all new fish — this brings out subclinical symptoms faster so you can identify and treat before main tank introduction
- ✦Keep a stock of salt, Metronidazole, and Praziquantel in your discus medicine cabinet — outbreak response speed determines survival rate