Although koi are usually gentle fish, they can become aggressive due to poor conditions, diet or unsuitable pond mates. ... Though koi are typically gentle and easygoing fish, they can at times be aggressive and may bully other fish depending on the circumstances.

Can koi fish bite you?

Are My Fingers Safe? Koi won't bite while you feed them, but may harmlessly pull on your finger as they try to get food. Koi have very muscular mouths and will be able to give your fingers a noticeable “pull” if they manage to get ahold of any while feeding.

Do koi attack humans?

It is a question as old as koi are, which is over one hundred years, and the answer is simple - yes koi do bite. However don't let that scare you away from feeding your koi by hand or sticking your toes in the water.