I had a similar problem with some pond goldfish, around 8 inches long. I
bought a medication from the pet store but I don't remember the name. It
was a dip that you mix up with water and then dip the fish in it for one or
two seconds and you do this two or three times in succession. I used an
extra large fish net to hold the fish as I dipped him/her into a baking dish
containing the medication mixture. Then release the fish back into the pond
and the problem is solved. You may have to repeat this but I don't think I
did. It was hard enough catching the critter in the first place. This all
took place twenty years ago so I am unfamiliar with the newest medications
but treating the entire pond will, hopefully, not be necessary. I had about
20 other fish in this pond and none of the others became infected. Good
luck.