I have about 10 goldfish and one large koi in a 1000 gallon pond. One of the goldfish developed what looks like a tumor on his head for over a year now but has seemed fine otherwise. However , Just recently we have noticed the other fish chasing it and nipping at the goldfish. This morning I went out and they had it cornered and were really going at it. I couldn't handle it any more and removed him to a temporary location until I figure this out

Is this a case of the weak one getting picked on?

I have also heard a lot about aggressive behavior towards a pregnant fish. Could the tumor be a moot point and this is just spawning behavior?

Thanks for any advice.