Why is my ranchu turning black?

Cause 1: healing ammonia burn

The most common reason a fish that was orange develops black smudges on the fins, edges of the scales or wen. Ammonia (or a sharp pH swing) burns the skin; as the burn heals, the tissue darkens, in the same way a scab does. The black appears days or weeks after the water problem, which is why keepers do not connect the two.

What to do: test ammonia and nitrite now. If either is above zero, do a 50% water change and keep changing daily until they read zero. If the water tests clean, the burn is already behind you; the black will fade over weeks to months as new skin grows. No medication is needed.

Cause 2: natural colour change

Young ranchu are bronze or grey-black as fry and turn orange, red, white or red-and-white during the first year. Some go through a mottled phase where black patches appear and vanish. Calico and black ranchu can also shift as they mature: a black ranchu often turns bronze or orange with age or in warm water, and there is nothing to be done about it. If the fish is under two years old and the water is clean, this is the likely answer.

Cause 3: black spot disease

Rare in aquariums. Tiny raised black dots caused by a parasite that needs snails and birds to complete its life cycle, so it arrives with pond-raised fish. It is harmless in a closed tank and the spots go away as the parasite dies.

Losing colour instead

A fish that fades from red to pale orange or white is usually short of carotenoids in its diet, kept in dim light or in very clean, colourless water, or simply ageing. Colour-enhancing food and a lightly green tub bring red back. White patches that look fuzzy or raised are not colour change; see diseases.