Wooden fish is a percussion instrument made of a hollow wooden block originally used by Buddhist priest to beat rhythm when chanting scriptures.
As for the origin of wooden fish, there is an interesting legend as follows:
Many years ago, a Chinese Buddhist went to India to acquire scriptures. One day, on his way to India, he found himself to be blocked by a flooding wide river. There appeared neither bridge nor boat. At this moment, a big fish swam up and back him across the river. In the middle of the river, the fish said to the Buddhist, 'Because I committed a crime, I have been sentenced to live in this river for many years. Now I am told that you spare no efforts to go to India for scriptures, so I come here to help you, just to atone for my crime. A good deed I do! If you meet Sakyamuni (the founder of Buddhism), please ask Him when I can become Bodhisattva'.
Being anxious to cross the river, the Buddhist accepted the fish's demand without hesitation. After having spent 17 years in India, the Buddhist went back to China, taking the scriptures he got. On the way back to China, he came near the former river, which was flooding furiously again. While he was worrying, the big fish appeared and gave him a hand again. In the middle of the water, it asked the Buddhist, 'You have been in India for many years. Did you ask Sakyamuni when I can become Bodhisattva?' The Buddhist replied, "Ah, Sorry! I forget'. On hearing this, the fish got angry. It vibrated itself only to get the Buddhist and his scripture books slide into water. A fisherman who happened to pass nearly helped him out of water, unfortunately, the scripture books were scattered by the flood.
The Buddhist came home, full of anger. He said to himself. 'It is the fish who makes my 17 years of efforts wasted.' Then he had a statue of fish head carved with wood, that is, wooden fish. When he recalled his adversity, he beat the wooden fish with a wooden hammer. To his surprise, each time he beat the wooden fish, the fish opened its mouth and vomited a character. He became so happy that if he had time, he always beat the wooden fish. A few years passing by, he got back what he had lost in water from the wooden fish's mouth.