Fish blocks are square-shaped blocks that are transparent. They have a light grey symbol of a fish on them. The blocks also have a light grey outline and are shiny, so they reflect a bit of light at the top left and bottom right corners.
In the game, the fish blocks are blocks that only the mystical flying fish can go through. If the raccoon touches the block, it will act like a normal wall (or platform) and the raccoon will not go through it. This makes it a hazard because the blocks may push the raccoon out of the bucket or the fish squeezes in areas the raccoon cannot reach.