My kohaku, Blondie, has been in my pond for 20 years. She seemed to lose buoyancy in early July, but otherwise has remained healthy. If there's the prospect of food, she comes swimming off the bottom and circles the pond. If it turns out there's no food or she gets fed she returns to the bottom, exact same spot (she's worn the algae off). Her fins are never clamped. She is strong. I've checked out her skin, but there are no problems, not even on her underside. It's like she accidentally swallowed a pound of buckshot. When not swimming hard, she just drops to bottom.

All the other fish are fine. No new fish have been introduced into the pond in 6 years. Pond ph is 7.2. Water quality is fine.

Oddly, this same thing happened last year, but I can't remember whether it was her or my showa. I was traveling and didn't pay it much mind. Within a month or so the fish was back to normal. Can anyone solve the mystery of the sinking fish?