Interesting question. Looking around the net there's really nothing on nitrile-butadiene safety and fish, or water exposure. I guess the only thing that pops up is that, like natural rubber, they can be broken down by oxidizers, like bleach. If you're just using them to keep your hands dry while scrubbing a full tank, and not using cleaning products, they'll probably be fine.
All cwt posts are implicitly appended with "just my two cents".