I have a comet that is 6 years old and has some type of growth under his gill. The other side is fine. For the winter, I keep him with 3 other goldfish (much smaller) in a 100 gallon tank with a light and submersed sponge box filter for a pond. Anyone have any ideas as to what it could be or what to do??
All aquarium levels check out fine. The water was just changed 75% less than a week ago.
I appreciate any advice! Thanks!!