Brook (Brooklynella)

Complete guide to identification, timeline, and treatment

What is Brook (Brooklynella)?

Brooklynella hostilis (Clownfish Disease) is a highly lethal ciliate protozoan parasite that primarily affects marine fish, with clownfish being especially susceptible. It attacks the gills and skin, causing rapid mucus production, tissue destruction, and death — often within 24–48 hours of visible symptoms if left untreated.

Symptoms

Excessive mucus sloughing off the body
Rapid or laboured breathing — gills heavily affected
Skin peeling or sloughing in patches
Lethargy and loss of appetite
Hovering near the surface or powerhead for oxygen
Rapid deterioration — death can occur within 24–48 hours

Causes

Brooklynella hostilis — a ciliate protozoan parasite
Introduction of infected fish without quarantine
Stress from transport, handling, or poor water quality
Clownfish are disproportionately susceptible
Overcrowding accelerates spread between fish

Brooklynella Life Cycle and Timeline

Hours 0–24
Parasite attaches to gills and skin. Fish may show subtle lethargy or slightly increased breathing rate. Often missed at this early stage.
Day 2–3
Rapid multiplication of parasites on gills and skin. Mucus sloughing becomes visible. Breathing becomes laboured. Fish deteriorates quickly.
Day 4–5
Severe gill and skin damage. Fish gasps at the surface. Skin peeling is pronounced. Death is likely without immediate aggressive treatment.
Day 6+
Brooklynella progresses far faster than most parasites — do not wait. Treatment must begin at the very first sign of symptoms.

Treatment Protocol

  • Act immediately — Brooklynella is fatal within days; do not delay treatment under any circumstances
  • Move all fish to a quarantine tank — leave the display tank fallow for 4–6 weeks
  • Administer a freshwater dip (pH and temperature matched) for 5 minutes to remove surface parasites
  • Treat with formalin — the most effective medication for Brooklynella
  • Perform daily 30% water changes in the quarantine tank to remove free parasites
  • Continue treatment for a minimum of 2 weeks; monitor closely for recurrence

Medication Options

  • Formalin (37% formaldehyde solution) — primary treatment; use as a bath or tank treatment per label instructions
  • Formalin + malachite green (e.g., Ich-X) — effective combination for Brooklynella
  • Freshwater dip — 5-minute dip in pH/temperature-matched freshwater; immediate relief for surface parasites
  • Chloroquine Phosphate — secondary option; less effective than formalin for Brook but useful in combination
  • Copper-based medications — less effective against Brooklynella than formalin; not recommended as primary treatment
  • Always quarantine new clownfish for 4–6 weeks before adding to a display tank — prevention is critical
Important

Brooklynella is one of the fastest-killing marine fish diseases. If you see symptoms, treat immediately — waiting even 12 hours can be fatal. Formalin is the most effective treatment but must be used carefully as it is toxic at high doses. Never treat in a reef tank. Quarantine all new marine fish without exception.

Important Disclaimer

The information provided in this guide is for educational purposes only. Always consult with a qualified aquatic veterinarian or fish health specialist before administering any treatments. Proceed with treatments at your own risk. We are not responsible for any adverse outcomes.