Why are my goldfish changing colour in my aquarium?
Goldfish are a domesticated form of a brown wild ancestor and their familiar bright colours are a result of many generations of selective breeding.
A glimpse into their original colour can be seen in many goldfish ponds when the dark brown fry are spotted amongst the plants in late summer. This camouflaged baby colour protects them from their many predators and as they grow larger they start to change colour. This tends to be linked to temperature and warmer conditions promote a much earlier shift from brown to red.
The genes responsible for this switch can lead to further colour changes and some of the more common ones are black fish turning red and red fish becoming silver with age.