What to do about a Koi fish bullying (biting) a black goldfish?
The black goldfish may have developed a weakness, a disease, or even old age, which you were not aware of. If it is only the one fish that would be my guess. If the larger fish keeps attacking others after this then it might be a problem with that particular fish and his/her territory. If you have had them together before and know that each of the different types of fish get along with each other that would be my guess. I use to raise birds and I kept many different types of birds together in one fly cage. If one became too territorial or aggressive I had to remove them. However if one became sick or weak they were an easy target for the stronger ones. Many times animals, fish, birds etc. attack the sick ones of their species. Even mothers will sometimes kill their own offspring if they have a birth defect or sometimes reject them if they are the wrong color. It is sad but if you watch a lot of nature shows it is a kind of mercy killing or even a kind of instinctual survival of the spe
