Begin by learning to draw a fish
Then use one primary colour to paint its face.
Use your second primary colour paint any extra fins, etc. Mix a SMALL amount of this second colour into your original colour. Paint one scale. Add another little but of your second colour. Continue, painting one scale after each addition, until you reach the tail.
Outline fish and scales with pastel. We used the colour we were adding, but black would work too. Mark in eyes and any scars or other features. Use green pastel to add in seaweed, and a limited palette of other colours to add background details such as bubbles (white) and other small fish.
Carefully wash over the outside of the picture with watercolour or blue dye.
Done! A fishy family :-)