White spot, also called ich or Ichthyophthirius multifiliis, is the most common disease in freshwater aquariums. It looks like the fish has been sprinkled with grains of salt. Treated early, it is very curable. Left alone, it can wipe out a whole tank.
How to recognize it
- Small white dots, about the size of a salt grain, on the body, fins and gills
- Fish rubbing against decor or the substrate (flashing)
- Rapid breathing if the gills are affected
- Clamped fins and loss of appetite in later stages
If the coating looks more like fine gold or gray dust, it may be velvet, which needs a different treatment.
Why it spreads so fast
The parasite has a life cycle with a stage on the fish and a free-swimming stage in the water. The white dots are parasites protected under the skin, where medication can’t reach them. They drop off, multiply in cysts on the substrate and release hundreds of new swimmers. Treatments only work on the free-swimming stage, so you must keep treating until every cycle is broken.
Treatment options
Heat (with or without salt)
- Raise the temperature gradually, 1–2 °C per day, to about 30 °C. This speeds up the parasite’s cycle.
- Add an air stone; warm water holds less oxygen.
- Optionally add 1–3 g/L of plain, non-iodized aquarium salt, dissolved first and added in portions.
- Vacuum the substrate and change 25–30% of the water every 1–2 days to remove cysts.
- Continue for at least 7 days after the last spot disappears, then lower the temperature slowly.
Coldwater fish (goldfish, koi, white cloud minnows) are not suited to the heat method. Avoid salt with corydoras, loaches, scaleless fish, shrimp, snails and live plants.
Medication
Use a white spot treatment, usually based on malachite green and/or formalin, at the dose on the label. Remove activated carbon, switch off UV and follow the label’s interval for repeat doses. Scaleless fish and some tetras often need a half dose, and invertebrates can be harmed.
Preventing it
- Keep new fish in a quarantine tank for 2–4 weeks.
- Avoid sudden temperature drops; stress weakens the fish’s defenses.
- Don’t move nets or water between tanks.
Saltwater fish get a different parasite, Cryptocaryon, which needs copper, hyposalinity or tank transfer instead. Fishtify’s quarantine mode includes day-by-day plans for both, with a dose calculator for your tank volume.