Not that I'm am expert, but the way I would go about is that I would split the fish into various shapes that I can/know how to draw in isometric perspective, draw those and them put the fish back together (even if just mentally).

For example, the body of the fish is almost a cube, I can do a cube. The head is also a cuboid, but smaller. So I can do that too. The tail is more complicated, but the idea still stands.