There's a phrase is Chinese called “人為刀俎，我為魚肉”，which means to be fish (meat) on someone's chopping block.
I think that if i were the fish, i wouldn't allow myself to be trampled.
Thus, I tied up the fish, it somehow looks like hanging (suicide) but it's above the knife and the chopping block, avoiding being killed by others.
Tying the suicide rope also looks like wearing a wound. And because of his shadow on the wall, the fish looks like having his life back. Thus, in the imagination of the fish, he has the freedom to swim back to the home he used to be.