Diagnose and Treat Saltwater Ich (White Spot Disease)

In this guide

Saltwater ich, also called white spot disease or marine ich, is caused by the protozoan parasite Cryptocaryon irritans and is one of the most common diseases marine keepers encounter, especially in newly introduced fish. Despite the similar name and appearance, it is unrelated to freshwater ich, which is caused by an entirely different organism, and the two require different treatments.

Recognising white spot

Small white cysts, roughly the size of a grain of salt or sugar, appear scattered across the body, fins and gills. Fish may lose their appetite, breathe rapidly as the parasite irritates the gills, and scratch against rock or substrate. Left untreated, the cysts eventually rupture, leaving open sores that invite secondary bacterial infection.

Life cycle, and why it matters for treatment

Cryptocaryon has four stages: a feeding stage attached to the fish, a reproductive stage where it drops off and encysts on the substrate, a stage where the cyst releases hundreds of new infective parasites, and a free-swimming stage that seeks out a new host. Medications only work on the free-swimming stage, which is why a single treatment never clears an outbreak. Treatment has to continue long enough to catch multiple waves of newly hatched parasites, typically 14 to 21 days.

Treatment options

• Copper based medication (copper sulfate or chelated copper), dosed and monitored precisely with a copper test kit, in a quarantine tank only, never where invertebrates or coral are present
• Hyposalinity, lowering specific gravity to 1.009 over 48 hours and holding it there for at least 3 weeks
• A UV sterilizer sized correctly for the tank volume, which kills free-swimming parasites as they pass through, useful as a supporting measure rather than a standalone cure
• A fine diatom filter, which can physically strain out the free-floating stage

Prevention matters more than treatment

Every new fish should spend a minimum of two to four weeks in quarantine before joining the display tank, regardless of how healthy it looks on arrival. Cryptocaryon is present in essentially all wild-caught marine stock at some level, and quarantine is the single most effective tool against it, far more so than any medication used after an outbreak has already started.

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