I want to use a balsamic glaze on fish - do I put it on the fish before I roast it or after?

It would help us to know what sort of fish you're glazing, what is included in the glaze, and how your intend to roast the fish.

Definitely after if you're roasting or grilling, since as Daniel said it'll burn unappetizingly. But, for future reference, if you're steaming or doing it "en papillote" it doesn't really matter.

Ii would glaze it only in the last few minutes. It's likely to burn if you keep it on the whole time.