Back to disease guide
parasite like

Brooklynella hostilis (clownfish disease)

A rapidly progressing skin ciliate that preferentially attacks clownfish (Amphiprion spp.) but can infect other marine fish. Causes excessive mucus, sloughing skin, rapid breathing, and fast deterioration. Often fatal within 24–48 hours without treatment. Can be confused with marine velvet but has a distinct slimy, sloughing appearance.

Urgentpattern match not diagnosis1 source note

Do first

  • Act immediately Brooklynella can kill a clownfish within 48 hours if untreated.
  • Move the affected fish to a hospital tank at once.
  • Treat with formalin (formaldehyde solution). This is the most effective treatment for Brooklynella.
  • If formalin is not available: treat with a combination product containing formalin and malachite green as a temporary measure.
  • Turn off UV sterilizer and remove carbon before treating.
  • Increase aeration formalin reduces dissolved oxygen.

Escalate if

  • Fish barely breathing or unresponsive acute hospitalization in formalin treatment needed immediately.
  • Skin sloughing extensively irreversible tissue damage may be occurring.
  • No improvement after 48 hours of formalin treatment re-examine diagnosis.

Water clues

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

ammonia above zero+3

Stress from poor water quality increases susceptibility correct water quality alongside treatment.

Care protocol

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

Differentiating Brooklynella from marine velvet

  1. Brooklynella: thick, slimy mucus coat; skin may appear to be sloughing or peeling; progresses very fast.
  2. Marine velvet: fine gold or rust-colored dust; intense flashing and scratching.
  3. Both cause rapid breathing, but Brooklynella tends to produce more visible surface abnormalities.
  4. Brooklynella primarily attacks clownfish. Other fish species are rarely affected as severely.
Cautions
  • Do not delay treatment trying to distinguish. Both are emergencies and formalin is effective against both.

Formalin treatment protocol

  1. Short formalin bath: 25 ppm formalin for 30–60 minutes in a separate, aerated container observe the fish constantly.
  2. Long-term treatment: 15–25 ppm formalin added to the hospital tank maintain with aeration.
  3. Water changes of 30% daily during formalin treatment to maintain water quality.
  4. Continue treatment for a minimum of 10–14 days.
  5. Monitor for sloughing of parasites and improvement in mucus production over days.
Cautions
  • Formalin is toxic to fish and humans at high concentrations measure precisely.
  • Formalin reduces oxygen significantly always increase aeration.
  • Do not use formalin in the main reef tank. It will harm corals and invertebrates.
  • If the fish stops moving during a formalin bath, transfer it to clean water immediately.

Source notes

References and context notes used for this triage entry.

Brooklynella hostilis Merck Vet ManualVeterinary reference for Brooklynella infections in marine fish.
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.

UrgentMarine velvet (Amyloodinium ocellatum)

The marine equivalent of freshwater velvet an extremely fast-moving dinoflagellate parasite causing gold or rust-colored dust on marine fish, rapid breathing, and quick death. Amyloodinium is considered one of the most dangerous diseases in the marine aquarium hobby and can kill an entire reef tank's fish population within 2–3 days if untreated.

High priorityGill flukes or body flukes (Dactylogyrus / Gyrodactylus)

Rapid breathing, gill flaring, flashing, and scratching caused by microscopic monogenean parasites on the gills (Dactylogyrus) or skin/fins (Gyrodactylus). Often introduced with new fish. Requires microscopic confirmation for certainty but may be treated empirically when symptoms are consistent.

UrgentVelvet (Oodinium / Amyloodinium)

Fine gold, rust, or yellowish dust-like shimmer on the body and fins, often with intense flashing and rapid breathing. Caused by the dinoflagellate parasite Oodinium (freshwater) or Amyloodinium (marine). Often missed until advanced because the coating is subtle.

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.