rocks in a pond are a no no. Get 'em out of there if you can as not only does it make cleaning difficult and a storage zone for decayed foods and poop, but a fish can absolutely hurt themselves. If your fish were spawning, that can be a pretty rambuncious activity, and thrashing is the norm. (Are you sure you saw spawning? It is a tad late in the season particularly where you are from. Not impossible tho, I guess).
The only other thing I can think of is hole-in-the-head disease as another possibility. It's definitely not common in pond fish (you see it more in indoor tanks), and it's probably unlikely this is what it is. H-i-H is caused by poor water quality.
Given that you are not sure if the fish has been hurt or if it's a parasitic disease such as H-I-H, I would personally treat the pond if it were my fish. Not sure if you have the ability to put the fish in a hospital tank, and if you don't I would feed a food like this
one. as a precaution. I use this food a couple of times of year as a preventative even tho my fish don't show any symptoms of anything.
