Question
QUESTION: I recently added a new koi to my pond. I have 3 koi's just over 1foot. As soon as I added the new koi (tad smaller) I noticed them chasing this poor koi. Upon closer inspection it has a lump on her left side of her belly believing that this koi is ready to lay eggs. The other koi are chasing, bumping, pushing and nudging her from her fins back. Is it possible that all 3 of my koi's are male and are "teaming" up on her this quick? We are currently hitting 30 degrees and my pond is quite warm. I have tons of plants, and my pond is roughly 2300 gallons. my water is fine and there is no other changes. I can't tell if they are males and if so I can't see anything on their fins or gills that would tell me they're ready.
Hope you can help me
Summer

ANSWER: It is possible that they are being aggressive for a reason other than spawning, but it sounds to me like you've described the situation exactly.

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QUESTION: What would other reasons be? My koi have never been aggressive like this before. And the koi I added does not show any signs of illness.

Answer
Fish, even koi, can be territorial, therefore aggressive towards newly introduced fish.  It is more likely spawning, though.  The male fish really don't know when to stop beating up on her, as they lack higher reasoning.  You can separate the new fish from the others with netting to avoid any injuries.  You can either do this immediately, or wait it out in hopes that this behavior is almost over.  It all depends on whether you want a lot of new baby fish soon.  If you are impartial, you should probably separate her.