Small white spots on the body and fins, typically the size of a grain of salt, often with the fish flicking against surfaces before the spots appear.
Short answer
Discrete salt-grain spots on the body and fins usually indicate whitespot. Several other conditions look similar, and treatment differs, so identify carefully before medicating.
Ordered by urgency, then by how commonly each one turns out to be the answer. The discriminator is the observation that separates this cause from the others — use it before acting.
How to tell: A fine gold or rust-coloured dust rather than discrete spots, best seen with a torch at an angle. Fish often clamp fins and breathe rapidly early.
How to tell: Distinct raised white spots the size of salt grains, scattered over body and fins, often preceded by flicking. Spots come and go as the parasite completes its cycle.
How to tell: Spots look tufted or fuzzy rather than like discrete grains, and often sit on damaged areas. Does not respond to whitespot treatment.
How to tell: White bumps on the gill covers and pectoral fins of a male cyprinid in breeding condition. Symmetrical, and the fish is otherwise entirely normal.
Checked before anything else, because a keeper who learns their swollen fish is carrying eggs and their spotted fish is a breeding male has been spared two unnecessary medications.
How to tell it apart: Small white bumps confined to the gill covers and leading edge of the pectoral fins, symmetrical, on males of danios, barbs, minnows and goldfish in breeding condition. Normal seasonal biology.
How to tell it apart: Look again from a different angle. Anything that shifts relative to the fish when you move was never on it.
Each entry states the observation that separates it from the others, and links to the full record. Work through them before deciding what you are treating.
How to tell it apart: Discrete, countable specks around 0.5–1 mm, raised, with sharp edges, spreading over days. This is the commonest cause and it is still not the only one.
How to tell it apart: Tufts with a slightly fuzzy edge, often larger and more irregular than whitespot and frequently centred on an existing sore. Does not respond to whitespot treatment — so treatment failure is the signal to reconsider rather than to treat harder.
How to tell it apart: A fine gold or rust dusting rather than countable spots, best seen with a torch along the flank in a darkened room.
How to tell it apart: Larger, irregular, cauliflower-textured growths developing over weeks rather than days, usually on fin margins.
How to tell it apart: A raised cotton-wool mass with filaments that stream in the current, at one site rather than scattered as specks.
AquariumHQ describes what is commonly reported and what husbandry correction is usually indicated. It does not diagnose and is not a substitute for an aquatic veterinarian. Many treatments are given to healthy fish suffering from a water-quality problem, which makes the outcome worse rather than better.