The focal length and brand have little to do with it. Nikkor/Nikon is the same thing, its Nikon brand. That lens will be ok at best on bright sunny day taking pics of your kids. I think it will struggle focusing on a fish in a tank. I would recommend at least a lens with a max aperture of 2.8. Your wife may have got you into a hobby more expensive than fish keeping! I looked at your tanks I think they are awesome and combining the hobbies can be very rewarding.