In what way do you mean "harmful"? It isn't toxic and it could provide valuable shade.

Not really - it is allelopathic, as I expect you know, and although not yet proven, in countries where parts of this plant are consumed, there is a much greater incidence of digestive tract and oesophogeal cancer which might be caused by consuming this plant. The spores are carcinogenic, but that's in the long rather than short term. Your fish are probably at more risk from any herbicide you might consider using to get rid of the bracken.

But bracken can be a menace if it gets established. I'd get it out and replace with a fern if you like the effect.