colorful yarn or tissue paper
Beforehand fold and tape the bottom corners securely, so the bottom of the bag looks like an arrow. This will become the head and mouth of the fish.
Fill your bags with colorful paper or tissue paper and seal shut.
Pinch and tie the bag near the opening with a string. The Ziploc end becomes the tail.
Draw a face with a marker and add a google eye.
Source: Kansai Kids