Hi Members ,
I have a back yard pond with 11 goldfish in it ,
I started with 5 and they have grown and bred .

1 of them has developed black spots on it's back and on its dorsal fin.
it appears happy and healthy and has not lost energy or appetite and none of the other 10 fish have it ...yet.

My pond is very self maintaining and is fed by rain off a shed roof , we have had rain recently and the water is good , I have a fountain in the pond which keeps the oxygen up.

Advise please .