Goldfish

[Charlesks]

goldfish losing colour

[rekstout]

If you keep them in a tank then some breeds will lose colour over time - try using different goldfish foods or pellets and check that your lighting is suitable.

[New Forester]

If they are breeding, the wild type will gradually reappear over several generations. These are brownish, much like other wild carp. I think wild "brownfish" are established in a number of places in Europe, perhaps including the UK.