One of the ways I design koi images now is by combining little sketches in different, somewhat random arrangements to discover which overall pattern of fishes provides the most appealing composition. In these small drawings the goal is to get a large pattern established. Particulars of color, surface, anatomy or generalization, paint texture, etc. will all be sorted out later.
The finished painting might turn out to be quite large. So I spend time giving the drawing ideas a good stare to imagine how the imagery would look if expanded into a much larger format.
The fish placement isn’t really much different from putting fruits on a table to draw still life or to decide the relative positioning of anything that will create two dimensional movement across the picture plane.