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internal parasite

Neon tetra disease (Pleistophora hyphessobryconis)

Progressive color loss (the neon-blue stripe fades or turns white in patches), body deformity, and wasting in small tetras and related species. Caused by the microsporidian parasite Pleistophora. Highly contagious, incurable, and almost always fatal affected fish should be humanely euthanized and removed promptly.

High prioritypattern match not diagnosis1 source note

Do first

  • Remove the affected fish from the tank immediately the parasite spreads when fish eat infected tissue.
  • Do not add the fish to another tank with healthy fish.
  • Euthanize humanely there is no effective treatment for Neon Tetra Disease.
  • Do not feed remaining fish for 24 hours and observe closely for signs in other tank inhabitants.
  • Disinfect any nets or equipment used with the affected fish.

Escalate if

  • Rapid spread through a school multiple fish declining within days of each other.
  • Uncertainty about whether it is NTD or Columnaris. Columnaris can be treated, so correct identification matters.
  • Owner distress about euthanasia decisions. These are legitimate situations where guidance helps.

Water clues

These readings can push this pattern higher or lower in the triage result.

ammonia above zero+1

Poor water quality can accelerate spread but is not the cause test anyway.

Care protocol

Follow only the steps that fit your species, tank inhabitants, and medication label.

Confirming neon tetra disease

  1. Look for bleaching or whitish patches on the body, particularly where the iridescent blue stripe should be.
  2. The patches are often irregular and progress over days to weeks.
  3. The fish may develop a curved spine (scoliosis) in advanced stages.
  4. Affected fish often become reclusive before visible color changes are obvious.
  5. False neon tetra disease (caused by Columnaris): similar color loss but faster progression and some response to antibiotics. It is not caused by Pleistophora.
Cautions
  • Do not confuse with simple ich (uniform white spots) or Columnaris (surface lesion with dry appearance).
  • If color loss is patchy and the fish is declining despite good water quality, NTD should be strongly suspected.

Tank management after confirmed NTD

  1. Remove and euthanize all visibly affected fish immediately.
  2. Observe remaining fish daily for the next 2–4 weeks.
  3. Do not add new small tetras to the tank during this monitoring period.
  4. The parasite can persist in the tank via fish-eating infected corpses remove dead fish immediately.
  5. If multiple fish are affected, consider whether the entire school needs to be humanely euthanized.
  6. After the outbreak, consider a full tank breakdown and disinfection before restocking.
Cautions
  • There is no proven medication that eliminates Pleistophora from a living fish.
  • Fish that appear to recover actually harbor the parasite and continue to be a source of infection.
  • Do not delay removal hoping the fish will recover.

Source notes

References and context notes used for this triage entry.

Microsporidian infections in fish Merck Vet ManualVeterinary reference for microsporidian parasites including Pleistophora.
Run Symptom CheckerCompare this pattern against water readings, affected count, and recent tank context.Ask AdvisorUse TankFlare Advisor to review this against your saved tank and recent logs.

Related patterns

Similar symptoms or risk factors worth comparing before treatment.

High priorityCamallanus worms (nematode)

Thin, reddish-brown worms visibly protruding from the fish's vent (anal opening). Camallanus are internal roundworms that embed in the gut lining. One of the few internal parasites visible to the naked eye. Highly contagious within a tank all fish must be treated, not just visibly affected individuals.

High priorityFish tuberculosis (Mycobacterium)

Chronic, progressive wasting with a combination of sunken belly, spinal deformity (bent spine), skin ulcers, and color loss. Caused by Mycobacterium marinum or related species. Incurable, highly persistent in tank environments, and importantly can cause skin infections in humans. Handle affected fish and their tank water with care.

Medium priorityInternal parasites / wasting disease

Progressive weight loss and sunken belly despite normal or increased appetite, often with abnormal feces (stringy, white, or mucus-laden). Caused by various internal parasites including Hexamita, Spironucleus, Capillaria, or other nematodes. Common in wild-caught fish and recently imported specimens.

UrgentKoi herpesvirus (KHV / Cyprinid herpesvirus 3)

A highly contagious and often fatal viral disease affecting koi (Cyprinus carpio) and common carp. Causes gill damage, lethargy, sunken eyes, and mass mortality typically at water temperatures between 16–25°C (60–77°F). A notifiable disease in many countries. There is no cure; management focuses on biosecurity and welfare decisions.