parasite · saltwater tank

Marine Ich (Cryptocaryon irritans)

Marine ich is caused by Cryptocaryon irritans, a ciliate parasite that produces white salt-grain spots on saltwater fish. Most common cause of fish loss in marine aquariums; typically introduced via unquarantined new fish.

Reviewed by the Fast Aquatics husbandry team · Updated May 2026

Severity: High - fatal if untreated within 2-3 weeks

Symptoms to look for

White salt-grain spots on fins, body, gills
Rapid breathing or gasping at the surface
Flashing or rubbing against rocks
Lethargy and reduced appetite
Cloudy or pale eyes
White stringy feces

What causes it

Almost always introduced via a new fish that skipped quarantine. The parasite has a free-swimming phase (theront) that can survive in a fishless tank for ~30 days before dying off.

Treatment options

Multiple effective treatments exist. Pick based on your tank type, livestock sensitivity, and severity. Always treat in a separate quarantine/hospital tank - most medications are toxic to coral, invertebrates, and live rock biology.

Copper treatment. Cupramine or Coppersafe at 0.35-0.5 ppm in a quarantine tank for 30 days minimum. Test copper levels every 2-3 days; copper degrades in the presence of carbon, so remove all carbon media. NEVER dose copper in the display tank - it kills coral, inverts, and live rock biology.

Chloroquine phosphate (CP). 60 mg per gallon for 30 days in quarantine. CP is gentler than copper and effective against multiple parasites simultaneously. Hard to source - check Reef2Reef vendor list.

Tank Transfer Method (TTM). Move infected fish to a clean tank every 72 hours, 4 transfers total. Disrupts the parasite life cycle (free-swimming phase needs >72 hours to find a host). No medication required - safe for sensitive species like wrasses or anthias.

Hyposalinity. Drop salinity to 1.009 SG over 48 hours and hold for 30 days. Effective for ich but stressful on fish. Not for inverts/corals.

Prevention

Quarantine ALL new fish for 30+ days minimum in a separate tank with copper or CP prophylactically. Treat any unquarantined fish that show symptoms immediately - ich spreads to every fish in days.

Frequently asked questions

What does Marine Ich (Cryptocaryon irritans) look like?

White salt-grain spots on fins, body, gills. Rapid breathing or gasping at the surface. Flashing or rubbing against rocks.

What causes Marine Ich (Cryptocaryon irritans)?

Almost always introduced via a new fish that skipped quarantine. The parasite has a free-swimming phase (theront) that can survive in a fishless tank for ~30 days before dying off.

How is Marine Ich (Cryptocaryon irritans) treated?

Copper treatment: Cupramine or Coppersafe at 0.35-0.5 ppm in a quarantine tank for 30 days minimum. Test copper levels every 2-3 days; copper degrades in the presence of carbon, so remove all carbon media. NEVER dose copper in the display tank - it kills coral, inverts, and live rock biology.

Can Marine Ich (Cryptocaryon irritans) be prevented?

Quarantine ALL new fish for 30+ days minimum in a separate tank with copper or CP prophylactically. Treat any unquarantined fish that show symptoms immediately - ich spreads to every fish in days.

How fatal is Marine Ich (Cryptocaryon irritans)?

High - fatal if untreated within 2-3 weeks

Should I treat in the display tank or quarantine?

Always treat in a separate quarantine or hospital tank. Most medications are toxic to coral, invertebrates, and live rock biology.

Browse the full disease database for 45 aquarium conditions with treatment protocols, or check the care library for prevention-focused husbandry guides. Use our symptom matcher to rank likely diseases from observed signs, the water parameter checker to diagnose related water-quality issues, or the QT timeline calculator to plan a treatment schedule.