Color of tropical fishes

Source: The Waltham Book of Companion Animal Nutrition, Pergamon Press, 1993
Tropical fishes come in many colors. Red, blue, purple, orange, yellow, silver and green occur in different combinations and graduations. These colors play a large part in the fishes' social conduct.
The building blocks of these colors in the skin and in the fishes’ flesh come from their feed. Many pigments are carotenoids. Over more than six hundred carotenoids are known. Plants and micro-organisms can synthesize carotenoids themselves, but animals cannot. They can, however, modify carotenoids through a biochemical route. The kinds of modifications depend on the species.
Typical colors of carotenoids range from yellow and orange to red. Complex carotenoids such as carotenoid lipoprotein, carotenoid protein and other derivates can expand this pallet of colors to blue, green and violet.

The carotenoids that are used most often for the pigmentation of fishes are astaxanthin and canthaxanthin. Goldfishes, for example, have to take in astaxanthin through their feed in order to keep their color orange. These fishes are not capable to produce astaxanthin themselves from beta-carotene.