Question
Some of my Koi seem to look like they are losing their orange scales and now have white skin in that place. One I found dead in the pond.They seem to still swim around as they are fine but I know this is not a normal skin or scales.  What do you think is wrong and how can I correct this before it kills all of my fish.
    Thank you

Answer
This could be the result of a few different things.  It could be a nasty external parasite.  The fish will sometimes rip their own scales off to get rid of the parasite underneath.  This could also be a bacterial infection, where instead of ulceration, you just saw a healed injury.  It could be a resuly of heavy ammonia.  Chances are though, that it is a physical injury, and not one resulting from microscopic organisms.  However, bacteria will see this raw area as their next meal, so monitor water quality very closely for a week or two.  Sometimes, scales will fall off in a way where they can grow back on their own.  Some Koi don't even have scales on parts of their body, so this isn't a huge problem.  If the area is inflamed, be more proactive in your solution.  You could even consider using some wound sealant powder.