The short answer
Small white, salt-like spots on a fish’s body and fins are most often ich (white spot disease), a common parasite. It’s usually set off by stress or a temperature swing, and it’s treatable — but the safe order of events is to check your water first, stabilise the tank, then research a proper treatment and get advice before dosing anything.
What white spots usually mean
The typical picture is tiny raised dots, like grains of salt, scattered over the body and fins, often with flicking or rubbing against decor, clamped fins and faster breathing. That combination points strongly to ich.
Not every white mark is a parasite, though — some male fish develop harmless breeding bumps, and fungus or excess slime can look similar. If the spots are few, isolated and the fish is behaving normally, watch closely before assuming the worst.
Safe first steps
Widely-recommended supportive measures for ich include raising the temperature gradually, keeping the water immaculate with regular changes, and using a recognised white-spot treatment. The right temperature, product and dose depend on your species and whether you keep plants or shrimp, some of which are sensitive — so research a treatment suited to your tank and consult a vet or experienced fishkeeper before dosing.
Please treat this as general information, not a diagnosis.
Stop it happening again
Ich thrives on stress. A cycled, stable tank, calm surroundings and a quarantine period for new fish are what keep it away — see do I need to quarantine new fish? For the full explanation of the parasite, read what is ich in aquarium fish?, and if you’re unsure whether your fish is unwell at all, see how do I know if my fish is sick?