Red is the most unobtrusive light to humans, sadly it also promotes angry behaviour (if you're into psychology).
Red is the 1 colour that is safe in darkrooms while working with reactive paper (not film tho). It is also the 1 colour that it takes your eyes less time to adjust back to dark. (this is the reason AFAIK that the military used red lens' on their lights, that way they wouldn't be at a disadvantage when they turned them off)..
So if Fishes eyes resemble anything close to humans, the red they will "see", but it won't bother them either way, (their eyes won't tire from red)
Blue on the other hand is a calming colour (again, if you're into psychology), it's also harder to focus on, hence harder to notice, I have to have every light off in my basement to see the moonilighting in my tank.
If you want to play with your eyes, open paint and make a picture with a black background, then start typing with different colours. you'll notice the red and blue are harder to read then green which our eyes are adept at picking up..
Either colour is acceptable, and both have their advantages and disadvatages (red looks so un-natural)
Just what I know of light with eyes.