I changed Jack's tank out today and a couple of hours after returning him to the tank, his left gill flap is extended and some tissue is extruding from the gill. The right gill flap is normal.
I removed him from the tank, removed the new items I'd added, replaced the water with new water and a bit of Epsom salts, and returned him to the tank. So far there's no change in the gill's condition.
Jack seems fine. He's swimming and exploring and ate a couple of pellets. If the gill weren't weird I'd say he's a healthy, happy fish.
Please see the picture below. The problem gill is in the red box.
HELP!?!?! What should I do for him???