HELP! over the past month my standard goldfish has lost its colour! for about two months now, one of my orange goldfish had been getting very pale. The fish is now bright white. there is obviously a problem but what is it!?

i have done tons of research and have found nothing! only information about fish getting darker! i have two other goldfish in the ten gallon tank and they are more than fine!
The ill fish's eyes are beginning to bulge out which if i'm correct is due to an internal infection which i will treat.
Should i quaranteine the fish whist treating?
it seems to be very poorly, just sits there in the corner staring at the bubble windmill.
I don't want my fish to die! i've had them for over two years now!

any help or advice would be most appreciated!
if anyone knows whats wrong with it that would be wonderfull so i can make it better!
HELP ASAP!