Blocking in the posture, proportions, and negative space body angles are the first steps in drawing a fish. You may see a parallel here to drawing birds, mammals, and everything else- it is the same system, just a different subject. Before you add any detail, block in these elements. If you add color with watercolor as in this demonstration, start with lighter values and work your way into the darks.
Click on the first image to start a step-by-step slide show.

Being careful not to smudge the dark lines, add more gray accents and details in the white part of the fish. It would have been easier to have done this before adding the black but sometimes you only figure these things out as the drawing moves along. The order here is because of a mistake, not an intentional sequence of steps.