Ich
Symptoms
Pinhead-sized white spots scattered across the body, fins, and gills — like the fish has been dusted with salt. Fish flash against decor (scratch sides of body), clamp fins, breathe rapidly, and lose appetite as the parasite multiplies. Gill infestations can suffocate before spots are visible on the body.
Causes
A ciliated protozoan with a three-stage life cycle: trophont (feeding on the fish), tomont (drops off, forms a cyst on substrate), and theront (free-swimming, infects new hosts). Outbreaks are triggered by stress — new arrivals, temperature swings, dirty water. The parasite is ubiquitous in freshwater systems and only blooms when hosts are immunocompromised.
Treatment
Raise temperature to 82-86°F if all your fish tolerate it (speeds the life cycle and exposes the vulnerable free-swimming stage). Treat the whole tank with an ich-specific medication — common active ingredients are malachite green + formalin, copper sulfate, or higher-dose aquarium salt (1 tablespoon per 5 gallons for scaleless-fish-free tanks). Run the treatment for the full course on the label — typically 10-14 days — even after spots disappear, because only the free-swimming stage is killed. Remove activated carbon while medicating.
Prevention
Quarantine every new fish for 4 weeks before adding to the display. Keep stable temperatures (avoid sudden drops). Don't move fish between tanks without disinfecting nets. Stress is the trigger — solid husbandry prevents outbreaks even when the parasite is present.
Notes
Scaleless fish (loaches, corys, eels, knifefish) tolerate salt and heat poorly. Use half-doses and watch closely. Tetras and similar small fish can be sensitive to copper-based meds — read the label.