Marine velvet (Amyloodinium) is a faster, deadlier parasite than ich — a fine dusty or velvety sheen on the fish with rapid breathing — that can wipe out a tank in days.
It moves far quicker than ich and demands immediate quarantine and treatment. Velvet is the single strongest argument for quarantining every new fish before it reaches your display. Triage, not diagnosis — act fast and get help.