whats the meaning of terrible fish?

The slippery and unnerving "fish" in the poem may represent that unavoidable, darker self that cannot help but challenge the socially acceptable self.

THE TERRIBLE FISH SEEN IN SYLVIA PLATHS POEM COULD REPRESENT THE AGING WOMEN WHO IS STRUGGLING TO HOLD ON HER PAST.OR WE CAN ALSO SAY THAT THE TERRIBLE FISH SYMBOLIZES THE WOMEN AGITATING AT THE LOSS OF HER YOUTHFUL BEAUTY.