Ich
Cryptocaryon irritans (saltwater) / Ichthyophthirius multifiliis (freshwater)Definition
Ich is the common name for two related parasitic protozoans that cause white spots resembling salt grains on fish. Saltwater ich is Cryptocaryon irritans; freshwater ich is Ichthyophthirius multifiliis. Both have free-swimming and cyst phases; treatment must target the free-swimming phase.
Practical use in aquariums
Saltwater treatment options: copper-based (Cupramine, Coppersafe) at 0.35-0.5 ppm for 30 days minimum, or chloroquine phosphate at 60 mg/gallon. Freshwater: heat (raise temp to 86°F for 14 days, only for fish that tolerate it), formalin/malachite green, or Ich-X. Always treat in quarantine, never the display tank with corals or invertebrates.
How Ich fits the bigger picture
Understanding Ich matters because it's connected to broader husbandry decisions: disease prevention is cheaper than treatment - quarantine + dipping protocols paid back the first time you avoid a tank-wide outbreak.
Browse the Fast Aquatics care library for full husbandry tutorials covering Ich in context.