White spots on fish (ich): freshwater and marine treatment

DiagnosticFreshwater tank; white dots, blisters or salt-grain spots on skin and fins; earlier, flashing, extra mucus and poor appetite. Gill-only infections may show no spots but kill fish quickly (UF/IFAS).

FixUF/IFAS: only the free-swimming theront is vulnerable, and at 75–79 °F the whole cycle takes about 3–6 days. Salt at 4–5 g/L as a prolonged bath for 7–10 days kills theronts (they can't tolerate 3–5 ppt, and most freshwater fish tolerate 5 ppt for weeks), but plants may not tolerate it; formalin at 25 mg/L (1 mL per 10 gallons), half that for sensitive species, is the other standard treatment. At warm temperatures treat daily for at least 3–5 treatments, and keep going until deaths stop. Don't use copper sulfate when alkalinity is below 50 mg/L. Formalin and copper are hazardous to people as well as fish: follow the product label's safety directions.

DiagnosticSaltwater tank; salt-grain white spots on body and fins, flashing; tangs are especially prone (UF/IFAS).

FixTreat in a hospital tank, never the reef display: copper held at 0.15–0.20 mg/L free copper for 3–6 weeks and tested at least twice a day, or the tank-transfer method, moving fish to a clean, dry tank every three days (UF/IFAS, Bulk Reef Supply). Copper kills most invertebrates, and calcium carbonate rock and sand bind it.

DiagnosticSaltwater fish sick or dying, sometimes before any spots are obvious; velvet can kill before spots appear (UF/IFAS).

FixCopper for at least 10–14 days (UF/IFAS); a freshwater dip alone isn't an effective treatment, and Bulk Reef Supply doesn't recommend tank transfer for velvet. Act fast.