My parents have an approximately 5,000 gallon backyard pond stocked
with goldfish and a few koi, maybe 8 fish in all. The weather has been
getting warmer, so they did the recommended spring mucking out today
and transferred all the fish to a 55 gallon bucket so the pond could be
drained. Then Mom noticed the largest of the koi (about 20") had a
strange, gray colored growth on the top of its head, about the size of
a dime. They asked me to come over and look at it since I have
aquariums (they're all sal****er except one freshwater tropical
community tank), so to make them happy I came over and had a look. This
fish is at least 10 years old and quite tame. He eats right out of your
hand, so they are really fond of him. I got to examine the fish when it
was in the bucket, and feel the lesion. It's right on the crown of his
head and almost looked like skin that had been peeled back, and some of
it came off as the fish moved around. The fish is orange and black, so
the gray bump on his head looks really odd. It didn't look like fungus
or ich to me. Maybe an injury? The pond has been set up for over a
decade, and this is the first time there has been any problems like
this. None of the other fish have any similar lesions. They didn't want
to stress the fish out too much, so the fish were all returned to the
pond once it had been refilled. What would be the best method of
treating the fish, since my parents don't want to lose him?