Tadpoles eating live fish

[Stevvy]

Hello, novices that we are, we took a plant from a nearby small river and put it in our pond around 5 years ago. Since then we have quite a lot of fish like the one in the photo. Unfortunately we have two of them now with a large white patch on their backs and the tadpoles are eating them whilst the fish is still alive. Anyone any idea what this white patch is and if it can be treated? Thank you.

 

[j.w]

@Stevvy
Could be a fungus. Not sure tho. Awful that the tadpoles are eating them