raingarden
Crowing
This happens all the time in goldfish, koi and some other fish. In fish, we know it is not a nutritional problem and is a very weakly heritable trait at best. It is physiological and, for whatever reason, they just stop producing and depositing melanin. Often, the melanophores are still there but they are empty. In fish it is occasionally reversable and they will regain the black pigment.
