I have a Canon Rebel XTI but that doesn"t matter here as it is all about the lens.
I have a EF100mm F2.8 Macro lens and I am sure there are similar ones for the Nikon.
I am NOT a pro for sure but I get quite some good results using the TV setting (shutter priority). For slow fish I shoot with 1/100 and faster fish like Tangs I go for 1/125 or 1/150.
Sometimes I use a tripod sometimes I don't.
When using the tripod I don't tight anything and make sure I can move the camera in all directions like up and down, left and right to follow the fish.
Here are a few shots taken without using the tripod.
One other major issue taking fish or coral pics is the "white balance". I tried the several WB settings on my camera and never got a real good result. Therefore I started shooting in the RAW format and this way you can adjust the WB with your photo software which is much better.
Once again I am just a beginner and I am sure there are better techniques but it works for me so far.
Below a few examples and more
here
Michael
MAXreefer