White spot — ich, to most keepers — is probably the first fish disease new aquarists learn to recognise, because its signature is written plainly on the skin. Each white grain is a single parasite that has burrowed into the fish's surface and is feeding there, protected under the outer layer of skin. It is common, it is contagious, and its life cycle produces one of the most misleading sights in fishkeeping: a fish that looks cured and is not.
The salt-grain spots
The grains are small, round and distinctly white, about the size of salt crystals, scattered over the body, fins and sometimes the eyes. Unlike a dusting, they are separate and countable — a scattering of pinpoints rather than a film. Numbers vary from a few grains on one fin to a heavy peppering over the whole fish. Look in good light and at an angle; on pale fish the spots are easiest to see against the fins, where they stand out against the light coming through the membrane. A close photograph makes a useful reference for whether numbers are rising or falling over the following days.
The vanishing trick of its life cycle
The stage you see on the fish is only one act of the parasite's life. The mature parasite drops off, settles on surfaces in the tank and divides into hundreds of free-swimming offspring that hunt for a fish to burrow into. Practically, this means the spots on a fish can disappear completely for a few days — prompting a relieved keeper to stand down — and then return in greater numbers as the new generation attaches. It also means the tank water and surfaces are part of the problem, not just the fish you can see spots on, and that other fish sharing the water are exposed even while they still look clean.
Behaviour and breathing clues
Well before the spots peak, the fish usually feel them. Rubbing and flicking against gravel, stones and decor — flashing — is the classic early behaviour, an attempt to scrape off an irritation. Fins clamp down, colours dull, and appetite becomes unreliable. The signs that matter most are respiratory: if the gills are carrying parasites, breathing turns fast and laboured, and fish hang near the surface or in the filter current where the water holds most oxygen. Small fish and fish already stressed can be overwhelmed quickly at that stage, which is why white spot with fast breathing is treated as urgent rather than routine.
Responding at the first grains
Because the multiplying stage happens off the fish, in the tank, timing and completeness matter more than intensity aimed at one fish: this is a whole-tank situation to discuss promptly with an aquatic veterinarian or fish health specialist, who can advise on the right approach for your species mix, since some fish and all invertebrates tolerate standard remedies poorly. Quarantine of new stock is what keeps most keepers from meeting this parasite at all. Record the day the first spots appeared, on which fish, in that fish's Pet Passport on KinsLe, and keep noting clears and returns; that dated pattern is exactly what the life cycle predicts, and it tells you and your specialist whether the outbreak is truly finished or merely between generations.