'Goldfish Variations, cleverly enhanced and titled by improbable.com'Goldfish Variations, cleverly enhanced and titled by improbable.comWhen
you go to the market to by fish, let's say salmon, do you pick out the
reddest salmon of the lot?  Most people do pick the reddest fish.
Therefore, fish color has an economic value.  Color also has an economic
value when it comes to pet fish, because people seem to choose the ones
with the brightest, strongest, clearest colors.  Here, a research team
from Akdeniz University in Turkey, attempts to change the color of goldfish.

purines, pteridines, and carotenoids are pigment groups that naturally
provide coloring to plants and animals.  Commercial fisheries, long
having recognized that those bright colorful fish sell, feed fish
synthetic forms of these pigments to enhance their marketability. Two
faculty members of the fisheries department of Akdeniz University
recently presented a paper at The International Conference of Agricultural Engineering describing on their work using real tomato as a source of carotenoid to make goldfish more orange.

researchers studied 255 goldfish separated into 3 groups based on their
skin pigmentation and fed them twice per day 2, 4, or 6 percent tomato
powder with their feed.  After 3 months, the tomato feedings were found
to be ineffective in changing the pigmentation of the goldfish.  No need
to try this experiment at home.


source: Investigation of the Effect of Tomato Powders as a Dietary Supplement on Skin Pigmentation of Goldfish via Improbable Research