The goldfish is very easy to fold. You can find the diagram here.
This model is folded from a rectangle. The diagram shows how to obtain a 4:3 (or 1.33:1) rectangle from a square paper, but it will also work with rectangles of slightly different proportions, like letter size or A4 paper proportions.
I folded the big goldfish with cookie (alfajor) wrapping paper, that is approximately 5 1/2" x 7 1/2". For the small ones, I used 3" squares of foil origami paper that I cut into 2 1/4" x 3"rectangles.