Question
I have a butterfly koi which I have had for 7 years in our 10,000.00 gal pond with one other koi and way to many goldfish. His coloration is orange with black spec and  white fancy very long tail with fancy white nose fins too he about 28" long.He had always been active swimming around and sunning himself by the waterfall and in between the water lilies. I purchased three more koi and he or she seemed really happy with the new additions he would chase or be chased and play with the others and they would all hang out together. Now he or she is not very active anymore spend all his or her time on the bottom of the pond and is only active for a short time in the morning jumping up like a whale and only eats very little not like before. His tail which use to be very white now has hues of reddish sometimes hues of orange. Not sure what is going on but sure wish he would go back to his old self soon. The other koi are not that big around 10" long they don't bother the butterfly koi so as to make him go hide or anything like that. The ph is good and the filter is in good working order all of the other fish are happy and go down and visit him from time to time but prefer to be on top of the water most of the time. Any clues as to what is going on would be great.

Answer
I often base my conclusions partially on sequence of events.  The sequence of events here doesn't point to anything concrete, but it does give me a few ideas.

My working theory is that the new koi came with some sort of bacteria, which has now transferred to the pre-existing fish.  This is normal and doesn't mean that the new fish are 'sick' necessarily.  The red streaks in the fins points to this.

The best thing to do is make sure there is no ammonia in the water, and observe whether the fish gasps at the surface for air.  Even if there is plenty of oxygen in the pond, a fish with something wrong will often still gasp at the surface.  Anything wrong with 10,000 gallons of water is not going to be an easy fix.  If you don't already have one, try using a pond aerator about halfway down in the water.