The pattern is not specific about the exact yarn or gauge. It recommends a “sport category 4-ply cotton yarn (red and orange)”.

In addition to the yarn and hook the pattern requires:
pair of plastic eyes (10mm)
tapestry needle
pins
stuffing

The book gives the following description:
<blockquote> A graphic and multimedia designer by day, Sanda’s eye for sharp design translates perfectly to her amigurumi creations. Crafting whimsical patterns with healthy doses of vibrant color, she loves designing projects for children and adults alike. This fish won’t fit in a tank, but will still make a splash with your family and friends!

Although it is not listed, you will also need some thread to attach the eyes.