We had 10 small (6") and 1 medium (11") koi in our 3 month old pond and just purchased 3 new large (13") koi. It's been 3 days now and the the new large koi are attacking the one original medium size koi. That one medium size koi does not appear sick but has always been a bit "lazy" in that it didn't aggressively go after floating fish food and it didn't much hang out with the smaller fish in the 4000 gallon pond. When the new larger fish were put in the pond it started being more social and active. but how its being relentlessly chased, attached, picked on, it appears they are trying to chew on her side near the gills for example.
And it appears now the other original smaller fish are beating up our victim too. I don't know it's sex but they seem to small to spawn and would the little ones be going after an older one for spawning purposes anyways? Could my victim fish be sick? Is this normal and they are trying to establish some sort of dominance or are they trying to kill off some competition?
Sure could use some emergency help here for fear of loosing a beautiful kohaku.