I dont know much about nikons, but you need to find out how to put it into shutter priority mode, on canon it is Tv mode. Then shut it to a high enough shutter speed your subjects no longer look blurred. For most fish tank photography I use at least 1/160th of a second or higher.
It will depend on your camera and lens if it can handle that shutter speed and still be letting in enough light, but try it anyway.