Agree with TS. The Canon 85/1.8 is a great lens (I own it), but probably not the best for fish (I'm assuming Nikon's 85/1.8 is similar). The AF is fast, but it's just a tad on the long side, and the focusing distance can be annoying when you want to be close to the tank.
Maybe try a 50 or 35mm if you're looking for a prime. The 50/1.4 should be a winner if it focuses quickly, not to mention a wonderful all around lens.