We tested the water and there is not now any ammonia. It tested zero.
We did however do a complete water change about 6 to 8 weeks ago so
maybe there was elevated ammonia in the water then.

Thanks for all the responses.

Bob

Robert Liles wrote:

Several of my goldfish are getting a black edge on the back of their
fins. Is this a sign of disease? Not all of the fish are affected. I
have just salted my pond if that affects the answer any.