why is the poet compared to a fish

The "Terrible fish " refers to the fear of a ageing that rises from the depths of her repressed mind, it also symbolizes the woman's self loathing and perception of herself as being loved and as old as a fish.
 

The woman is compared to a fish because on looking from above the surface, a fish inside water appears ugly and shapeless. However, if one goes inside the water and looks at it, it looks beautiful. Similarly, the woman looks at her outward appearance in the mirror which is unattractive and hard-featured. But on delving deeper, there is another facet to her personality, that is her inner beauty, beauty that is not skin deep. The poetess wants to emphasise that one should look out for innate qualities rather than outwardly appearances.