First draw a square
- add a triangle front and back
- add a tilted triangle tail

- draw the fins
- add an eye
- draw the mouth, a loveheart shape on its side

- Draw wavy up-and-down stripes

- erase parts of your squares and triangles to make the body parts join up as in this picture. Round off the tail and body corners. You might like to trace it onto a fresh piece of paper.

Here's the fish with a touch of color quickly added with a basic paint program. It just took a couple of minutes with using the flood-fill bucket to color the stripes, and sweeping the lighten and darken tool across the fish with a large brush to add highlight and shadow.