In the wild fish do get injured and they do recover. The perennial debate over whether fish can feel pain rages on. But if asked, I feel they want to live. So let it be and see if it survives.
They can ask you almost anything, but you can leave it blank if you don't wish to tell them. It may prejudice your application though. Medical conditions are usually confidential except on things such as insurance forms. Of course, if someone is applying to be a steeple-jack, they have to say if...