Behold, he that keepeth Israel shall neither slumber nor sleep. Psalm 121:4
A fish's eyes never close, so they are taken to represent the watchful care of our Heavenly Father for his children. Ellen Frankel tells us more:
"Because fish dwell in the depths, they have traditionally been associated with awe and mystery.
Fish are also associated with fertility because of the vast quantity of eggs they lay. When Jacob blesses Joseph’s two sons, Ephraim and Manesseh, he says: “Let them multiply in the midst of the earth.” The Hebrew word for “multiply”—ve-yidgu—is derived from fish—dagim. …
The rabbis pointed out that just as the sea protects fish from the Evil Eye by covering them, so too the Evil Eye has no power over the “seed of Joseph,” the Jewish People, because Jacob blessed Joseph’s two sons, and all their descendants, by comparing them to fish (Berakhot 20a)."
(Frankel, Ellen, and Betsy Platkin Teutsch. 1992. The Encyclopedia of Jewish Symbols. Northvale, NJ: Jason Aronson Inc., pg 55)