Marine Ich (Cryptocaryon): symptoms, treatment, prevention

Cryptocaryon irritans is an obligate ciliate parasite of marine fish producing white spots on skin, fins and gills. Untreated outbreaks are usually fatal.

Overview

Marine Ich is caused by Cryptocaryon irritans, an obligate ciliate parasite of saltwater fish. The life cycle is slower than freshwater Ichthyophthirius, which makes outbreaks easier to miss but requires extended treatment courses. Mortality is very high without intervention.

Symptoms

White spots on body and fins
Rapid gill respiration
Scratching on rocks
Hiding
Cloudy eyes
Weight loss

Causes

Infection typically enters the system with new fish, contaminated water, or shared equipment from an infected tank. Stress, fluctuating salinity or temperature, and crowding allow the free-swimming theront stage to establish.

Diagnosis

White spots on a marine fish that persist longer than in freshwater Ich, combined with rapid gilling and scratching, are characteristic. Definitive diagnosis is by microscopy of a skin scrape revealing rotating ciliated trophonts; rule out marine velvet (finer dust) and Brooklynella (heavy mucus sloughing).

Treatment

Treatment must address the free-swimming theront stage in the water column; trophonts on fish are protected. The display tank cannot be effectively treated without harming invertebrates, so all fish are moved to a separate quarantine system for the duration of the course.

Quarantine

Set up a bare-bottom quarantine tank with mature sponge filter, heater and aeration. Move every fish from the display; leave the display fallow without fish for at least eight weeks at normal temperature so the parasite life cycle completes and dies out.

Medication

Hyposalinity: lower specific gravity to 1.009 over 48 hours and hold for 8-10 weeks in quarantine; unsafe for invertebrates and live rock.
Copper sulfate or chelated copper (copper-EDTA) maintained at 0.5 ppm in a bare quarantine for at least 30 days, with daily copper testing; lethal to invertebrates.
Chloroquine phosphate at 40-60 mg/L as a single dose lasting about 30 days in quarantine; veterinary prescription in many regions.

Recovery

Continue feeding high-quality enriched food during treatment to support immunity. Hold fish in quarantine for the full course even if visible spots disappear, then transfer back to the fallowed display after a final inspection.

Prevention

Mandatory 6-week quarantine for all new fish
Tank-transfer method for sensitive species
Stable salinity and temperature
UV sterilizer as adjunct