| What It Is | Cryptocaryon irritans parasite |
| Looks Like | Grains of salt on fins & body |
| Danger | High — can wipe out a tank |
| Treatable | Yes, in a bare quarantine tank |
| Reef-Safe Cure | None — must treat in QT |
Overview
Marine ich (Cryptocaryon irritans) is the most common fish disease in the hobby. It’s a parasite that shows as tiny white “salt grain” spots on fins and body. The tricky part is its life cycle: the visible spots are only one stage, and the parasite drops off to reproduce in the sand before reinfecting — so “the spots went away” does not mean it’s gone. Tangs, and stressed or newly-added fish, are especially prone.
Symptoms
Pinhead-sized white spots like grains of salt or sugar on the fins, body, and gills; flashing (scratching against rock); rapid breathing; clamped fins; and lethargy or hiding. Spots appear, then vanish as parasites drop off — then return in greater numbers a few days later. That come-and-go cycle is the signature of ich.
How to Treat It
- Move fish to a bare quarantine tank — you cannot safely medicate a reef; the effective treatments harm corals and inverts.
- Copper (at therapeutic level, monitored with a copper test) or the tank-transfer method (moving fish to fresh clean tanks on a schedule that outpaces the parasite’s life cycle) are the reliable cures.
- Run the full course — treat for the complete recommended duration, not until spots disappear, because of the life cycle.
- Leave the display fishless for around 76 days so the parasite starves out with no host, while treated fish recover in QT.
Prevention
Quarantine every new fish — ich almost always rides in on a new arrival. Keep fish unstressed with stable parameters, good food, and no bullying; a healthy fish’s slime coat resists infection. A proper quarantine routine is the single best defense against ever seeing ich in your display.
Common Mistakes
“Reef-safe ich cures”: most don’t reliably work — there is no proven in-display cure; treat in QT. Stopping when spots vanish: the parasite is mid-cycle, not gone — finish the course. Not quarantining new fish: the reason ich enters a tank in the first place.
Related Guides
Why You Need a Quarantine Tank · Marine Velvet: The Fast Killer · Yellow Tang
Frequently Asked Questions
What does marine ich look like?
Tiny white salt-grain spots on the fins and body, along with flashing (scratching on rocks), rapid breathing and clamped fins. It is caused by the parasite Cryptocaryon irritans.
How do I treat marine ich?
Move affected fish to a quarantine tank and treat with copper at therapeutic levels for the full course, or use the tank-transfer method. Reef-safe additives rarely cure it; proper medication in quarantine does.
Can I treat ich in my display tank?
No. Copper and most effective medications kill inverts and corals, and the parasite also has a stage in the substrate. The reliable path is treating fish in a separate quarantine tank and leaving the display fishless for 76 or more days.