Fungal infections announce themselves on the surface, which makes them one of the easier fish problems to spot early — if you know the texture you are looking for. True fungal growth on a fish looks like cotton wool: soft, raised, slightly fuzzy tufts in white, grey or occasionally grey-green, quite different from the flat sheen of normal colour or the smooth film of excess slime. The growth is made of fine fungal threads feeding on damaged tissue at the skin's surface.
Where the patches start, and why there
Fungus is an opportunist. Its spores are present in most aquarium water, but they can only gain a foothold where the fish's slime coat and skin are already broken — a scrape from sharp decor, a nip from a tank mate, a wound from netting or handling, or skin weakened by poor water quality. So the first tuft typically appears at an injury site: a flank, the edge of a fin, the lips, or around the base of a damaged scale. A patch appearing with no visible injury is still possible where the fish's condition has been run down by stress or another illness first.
How it behaves as it develops
Left alone, the tuft thickens and spreads outward across the skin, and in advanced cases patches merge into larger grey mats. The fish's behaviour changes alongside: fins clamped against the body, rubbing or flicking against hard surfaces, hanging listlessly, and a fading appetite. Growth near the gills is the most serious development, because it interferes with breathing — a fish gasping at the surface with a patch anywhere near the gill cover needs prompt attention rather than watchful waiting. Fungus on eggs in a breeding tank looks similar, spreading from infertile eggs to healthy neighbours.
The lookalike that changes the answer
The catch is that one of the commonest bacterial infections of fish produces pale, patchy growth around the mouth and body that the eye can mistake for fungus almost exactly — yet the two problems are managed differently, and guessing wrong wastes the days when action works best. Texture helps a little: bacterial patches tend to look flatter and more like worn, pale skin, while true fungus stands up in threads. But even experienced keepers treat home identification as provisional. That is why the honest answer to "is it fungus?" from a photograph is often "it is one of two things," and why an aquatic veterinarian or fish health specialist, sometimes examining a gentle skin scrape, is the route to certainty.
What to do when you spot a tuft
Check the water first — ammonia, nitrite and general cleanliness — because fungus exploits the conditions that poor water creates, and no treatment outruns a tank that keeps injuring its fish. Note sharp decor, bullying tank mates and any recent handling that could have opened the skin. Photograph the patch and record its size, position and the date in the fish's Pet Passport on KinsLe; a dated sequence turns "it looks bigger" into something you can actually show a specialist, and it tells you objectively whether whatever you and they decide to do is winning.