one koi dying?

Question
Hello, we have a 9' x 16' pond with a 9' waterfall.  We have a total of 25? fish, about 6 of those are gold fish and about 12
more are babies from last fall.  We have 3, 14" koi and most of the rest are about 6" - 9".  The pond is 1/2 shade, 1/2 sun.  The pond is about 2000 lbs.  We tested our water, everything seems to be great, however, this past weekend one of our favorites, a butterfly koi seemed to be laying on the bottom.  Then, we had to get it out of some rocks that it seemed to be jammed into.  I know sponing season is upon us, but, this is different!  This morning she floated into the skimmer, when my husband pulled her out, she swam off!  The other fish are pushing the butterfly koi out of the pond and eating at it's fins.  The butterfly koi know had red fins and has tears? on the tail fin and on top of it's head.  We have removed the butterfly koi from the pond in hopes to save it, it doesn't look good though!  What has caused this to happen and why only this one fish?
Amy

Answer
Dear Amy,

Thank you for your e-mail.
When water is fine and season is starting up also bacteria and parasites are tend to grow and expand.
The koi laying between rocks maybe an attempt to lay straight and to get a silent place.
This can mean it's (very) sick.
If waterquality is ensured to be fine the next thing to do is to observe the koi.
Is it also flashing (rubbing) against rocks, bottom are maybe jumping?
This intend to be the cause of Parasites, waterquality...
If it is laying still it is the same, also bacteria is possible.
Only way to know is to check the slime and check it by microscope.
Tears are more likely to be parasites or extensive slimeproducement.
http://users.skynet.be/eddy.keuninckx/wittestip3.JPG
http://www.koikarper.nl/images/koi/ziekten/Lernaea.jpg
http://members.home.nl/fredskoisite/koiziekte_bestanden/koiziekten.jpg
There are product available against parasites or bacteria.
When you do not want to go to a vet or expert for the microscopy then you can start using a broad spectrum anti parasite product and medicate and watch them in quarantine.

Good luck,
Regards,
Raf