Marine White Spot (Cryptocaryon irritans): How It Differs From Freshwater Ich
White spots covering a marine fish's skin and fins look a lot like freshwater ich, yet the responsible parasite, Cryptocaryon irritans, is a distinct species from Ichthyophthirius multifiliis and doesn't respond to the same treatments. A review by Colorni and Burgess published in 1997 in Marine and Freshwater Behaviour and Physiology (research on Google Scholar) details this marine parasite's life cycle and the treatments that work against it. Find our other pointers in the care and grooming section and on the fishkeeping hub.
Why doesn't the freshwater treatment work in a marine tank?
Freshwater white spot is often treated by raising the temperature and adding salt, two levers already permanently present in a marine tank and therefore ineffective against Cryptocaryon irritans. This marine parasite instead responds to a controlled-dose copper treatment, a substance toxic to invertebrates that rules out any direct treatment in a reef tank stocked with corals or shrimp.
How do you treat it without wiping out the reef's invertebrates?
Copper treatment must always happen in a separate hospital tank, never directly in the main reef tank, isolating the affected fish for the duration of treatment. That's one reason a dedicated hospital tank stays essential once you're keeping marine fish alongside corals.
How do you monitor the copper dose during treatment?
A dedicated ionic copper test, separate from your usual water tests, needs to be run daily during treatment: too low a dose won't kill the parasite, too high becomes toxic to the fish itself.
Does quarantining new arrivals still help against this parasite?
Yes, just as with freshwater, the vast majority of infections arrive with a newly purchased fish; observing a new arrival for several weeks in a hospital tank before adding it to the reef tank sharply limits the risk of introducing Cryptocaryon irritans among corals you can't treat directly.
- Marine white spot is caused by a distinct parasite from the freshwater one, Cryptocaryon irritans
- Raising temperature and adding salt, already present in marine tanks, aren't enough to treat it
- Copper treatment in a separate hospital tank is the reliable method, never in a stocked reef tank
- Quarantining new fish sharply limits the risk of introducing the parasite
Frequently asked questions
Can you treat directly in the reef tank by temporarily removing the corals?
That's not recommended: copper soaks durably into decor and substrate, which makes the tank unsuitable for invertebrates long after the visible treatment ends.
How long does copper treatment against this parasite take?
Generally several weeks at a stable dose, long enough to cover several of the parasite's reproductive cycles; stopping too early risks a relapse.
Can a marine fish develop immunity after an infestation?
Some partial resistance has been observed in surviving fish, but it remains insufficient to skip treatment in the event of a new confirmed infestation.
This guide is part of Planète Pets’s Fishkeeping universe. Our advice is general in nature: for any health concern, your veterinarian remains the only reference.