What healthy gills and skin look like
An axolotl's glory is its gills: three stalks on each side of the head, fringed with fine filaments that wave gently as water passes over them, giving a full, feathery silhouette. On a healthy animal the filaments are dense and even, the skin is smooth with a light glossy slime coat, and the colour — pinkish on a pale leucistic animal, dark on a wild type — is consistent across the body. Learn that normal picture on a good day, because fungal problems show themselves as departures from it. Photograph the gills from the front monthly, or whenever something catches your attention, so you are comparing against evidence rather than a memory of how fluffy the gills used to be.
The patches and the gill changes
Fungus on an axolotl looks like something growing on the animal rather than a change in the animal itself: white, grey or faintly fluffy tufts, like tiny wads of cotton wool, attached most often to the gill stalks and filaments, and sometimes to the tail tip, toes or a patch of skin. The filaments beneath a tuft are being damaged, so an affected gill looks frayed, thinner or shorter than its pair on the other side — comparing left and right is one of the most reliable checks you have. As filaments are lost the whole gill shrinks and pales or reddens at the base. Distinguish this from the normal slime coat, which is smooth and even, and from shed skin or debris, which moves loosely in the water rather than clinging and growing in place day after day.
The behaviour that goes with it
An axolotl with irritated, damaged gills breathes and behaves differently. The gills may flick forward repeatedly, a sign of irritation or poor water rather than the occasional lazy flick of a resting animal. Appetite is usually the next casualty — food refused, or taken and dropped. Some animals float oddly, hang at the surface, or become abnormally still on the bottom; others grow restless. None of these behaviours names the problem on its own, but behaviour change alongside a visible patch is a complete sentence: something is wrong on the surface of the animal and it is costing the animal energy. Note the day each change appeared, because the speed of spread is information a vet will use.
Water is part of the picture, every time
Fungus is an opportunist: it colonises axolotls whose defences have been lowered, and the usual culprit is the water. Axolotls are cold-water animals that do best in cool conditions, roughly the low to mid sixties Fahrenheit; water that runs warm stresses them and encourages fungal growth. Ammonia and nitrite from an overloaded filter or uneaten food decaying in the tank burn gill filaments and open the door. So the moment you spot a patch, test the water and read the thermometer before you do anything else, and correct anything that has drifted with a partial water change using dechlorinated water matched for temperature. Record the readings. If the water turns out to be the trigger, finding that out is as valuable as any treatment, because fungus treated in unchanged water simply returns.
When to involve a vet
A single small tuft on a bright, eating axolotl in water that tests clean can be watched closely for a day or two while conditions are perfected — but watch actively, with photographs, not hopefully. Escalate to a vet who treats amphibians or aquatic exotics if the patches spread or multiply, if a gill is visibly shrinking, if the animal stops eating, or if redness and rawness appear where a patch was. Avoid reaching for aquarium remedies at random; axolotl skin is exceptionally permeable and many fish products are unsafe for them, so product decisions belong in the vet conversation. Bring your readings and photographs: filed in the axolotl's KinsLe Pet Passport, the dated water tests and gill pictures show the vet the outbreak's timeline and its likely trigger in one view.