This is my CG goldfish. I used Photoshop techniques of digital painting and textures on top of a 3D Model.
This 3D model is from Adobe Stock Images. I took off the shadow, changed the lighting, and squished out my silhouette with Liquify and Transform Warp in Photoshop. For the final steps, I used a fish scale photo for the scale textures, and created the fin textures with the Gradient Editor. The head is a texture I created that I use often for film grain. Other than that it was just a matter of painting away!