It is difficult to say precisely what is
happening to your fish because koi can change
coloration for many reasons. Most of the time,
color changes in koi are not a concern; part of
the challenge for koi hobbyists interested in
color is to figure out the right combination of
diet and genetics to produce the most beautiful
Koi obtain the pigments required for
coloration through their diet. Orange and red
colors are a result of the deposition of pigments
(called Erythrin) in fish cells called
chromatophores. Perhaps your fish have been
consuming more Erythrin-rich foods, such as algae,
than they had previously, which may be filling
their chromatophores with red and orange pigments.
It is also possible that the chromatophores have
actually filled completely, and the fish are
passing the excess pigment through their feces.
When this happens, white parts of the fish become
pinkish due to a build-up of red pigments on the
outside of the fish. This pigment is NOT in the
chromatophores and should disappear if the amount
of color food given is reduced.
the following website for a very thorough
discussion of color changes in koi: color_changes
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