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Marine ich (Cryptocaryon irritans white spot disease)

The marine equivalent of freshwater ich white salt-grain spots on skin and fins, with flashing and rapid breathing. Caused by the ciliate Cryptocaryon irritans. Nearly ubiquitous in marine aquariums and one of the most common diseases in reef tanks. The display tank must go fallow while fish are treated in a hospital tank, because Cryptocaryon cannot be treated safely in the presence of corals and invertebrates.

High prioritypattern match not diagnosis2 source notes

Do first

  • Quarantine protocol: move ALL fish from the display tank to a hospital/quarantine tank.
  • Do not treat the reef or display tank with copper or other antiparasitics. This will kill invertebrates and corals.
  • The display tank must go fallow (fishless) for a minimum of 72–76 days at 25°C to break the Cryptocaryon life cycle.
  • Treat fish in the hospital tank with therapeutic copper or chloroquine phosphate.
  • Perform a hyposalinity protocol (if all fish species tolerate it) as an alternative to copper.

Escalate if

  • Fish gasping severely or showing rapid gill movement gill involvement is life-threatening.
  • Fish dying in the hospital tank despite treatment verify copper level and temperature.
  • No improvement after 7 days of correct treatment.

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 to Cryptocaryon.

temp above species max+2

Warmer temperatures speed up the Cryptocaryon life cycle.

Care protocol

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

Fallow tank protocol

  1. Remove all fish from the display tank even fish that appear healthy are assumed to be infected.
  2. Maintain the display tank with corals, invertebrates, and normal husbandry just without fish.
  3. Run the fallow period for a minimum of 72 days at 25°C (77°F) longer at cooler temperatures.
  4. Raise display tank temperature slightly (if safe for corals) to speed the life cycle and shorten the fallow period.
  5. After fallow period: return healthy, treated fish to the display tank.
Cautions
  • A single fish left in the display tank restarts the clock all fish must be removed.
  • Invertebrates can carry tomonts (cysts) the full fallow period cannot be shortened.

Hospital tank treatment

  1. Set up a bare-bottom hospital tank substrate harbors tomonts.
  2. Treat with therapeutic copper at 1.5–2.0 ppm free ionic copper, verified daily with a copper test kit.
  3. Alternatively: chloroquine phosphate at 40 mg/L effective and less stressful on sensitive fish.
  4. Hyposalinity (1.009 SG) is an alternative for fish that cannot tolerate copper or chloroquine not as reliable for advanced cases.
  5. Maintain treatment for at least 30 days in the hospital tank.
  6. After treatment, the fish must still wait for the display tank fallow period to complete.
Cautions
  • Copper test kits are required dosing without testing is dangerous.
  • Copper is not safe for scaleless or particularly copper-sensitive species use chloroquine phosphate instead.
  • Do not cross-contaminate nets or equipment between hospital and display tanks.

Source notes

References and context notes used for this triage entry.

Cryptocaryon irritans treatment Merck Vet ManualVeterinary reference for marine ich.Marine Ich and Velvet Treatment Reef2ReefCommunity reference verify all treatment protocols against veterinary sources.
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.

UrgentBrooklynella 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.

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.