Applying scales to the fish
After giving Mr Beef some bling, I use lines to paint a fish scale pattern on a new layer (as shown). Next I use Edit> Transform>Warp to wrap this pattern around the fish. Then I Cmd-click the layer thumbnail to load it as a selection. Now I invert the loaded selection and, on a new layer, run a soft wide white brush over the middle of the fish. Instant fish scales!
Glows and colour overlays
I create a layers folder called Glows, then on a layer in it I airbrush turquoise on the lower torso and lighter colours on the shoulders. I set the layer to Overlay and 50 per cent Opacity. Next layer I airbrush white on sunlit areas, repeat on a layer in yellow, and set both to Overlay to add glowing translucency to the skin. On another layer I add subtle water reflections on the lower abs.