Can anyone diagnose what is exactly wrong with one of our goldfish.

1 of our 15 fish is showing some wierd behaviour. About 2
months ago it turned completely silver ( took around a fortnight to happen)
....
the fish was pale orange to begin.
Although it's feeding normally, with the others & swimming just as fast,
with
them, it appears to have lost all sense of co-ordination, swimming /
flipping
around in circles & swimming upside down sometimes. My wife suggested that
it may be *blind* ( ?? ) .... but, it reacts to movement outside the tank,
just like
the others.

Any ideas ?