One of my pond goldfish seems to have a serious problem.

It is very lethargic and swimming in tight circles and seems to be using only one fin only to guide it. The other side seems completely paralysed. In addition the dorsal fin seems quite tatty in that the fin appears to separating. I have also noticed this on the ventral fins.

Can anybody advise what might be happening as I have a number of other fish including Koi that I do not want to be infected.

It also happens that this weekend ,I there has been a large anmount of mating in the pond. Could the fish just be exhaused and in bad condition due to being chased around the pond by the males in the pond???

Grateful for any advice

John