In Parshas Vayechi, the Torah describes the bracha that Yakov Avinu gave to Yosef's sons Efraim and Menashe. There are two important points that require explanation. The first is that Yakov says that the Jewish people will always bless their children by saying that Hashem should cause them to be like Efraim and Menashe. What is the deeper significance of this? Why do we specifically bless our children this way? It is also interesting to note that when Yakov gives them their bracha, he says that they should be like fish, multiplying greatly in the land. Why does Yakov refer to fish in this bracha?
The Maharsha in Sanhedrin (12A) gives a very beautiful explanation of this passuk. He points out that there are twelve tribes in the Jewish people, and a thirteenth hidden tribe, when Yosef splits in two. Still, when the tribes are counted, there are always twelve, because when Levi is counted, Yosef is counted as a single tribe; whereas when Efraim and Menashe are counted separately, Levi is not included in the twelve. Similarly, the Maharsha points out, there are twelve months in the year, with a thirteenth 'hidden' month during the leap year. The month that is always repeated is the month of Adar, whose mazal is the fish. Here we have the theme of fish being repeated again. What is the deeper message here?
The concept of the hidden number the thirteen, and the fact that it corresponds to fish, hints to the idea that beyond the reality that we see, there is a deeper root of spirituality animating that reality. Just as fish remain beneath the surface, too numerous to count, so too the spiritual realm is completely hidden, yet it is the multi-faceted life force at the root of all we experience.
This perfectly corresponds to the character of Yosef, who was hidden in Mitzrayim, appearing to be no more than all of the Egyptians for whom he was a leader. And yet, beneath the surface of this Egyptian ruler was a son of Yakov, who remained faithful to the Torah of his father. It was this faithfulness, despite the dangerous spiritual waters of Egypt, that earned him the first-born rights that resulted in his double portion in the Jewish people. Efraim and Menashe were the 'fish' who represented the spiritual power of Yosef, one who is hidden beneath the surface, and at the same time is the spiritual force guiding and supporting his brothers, both spiritually and physically, during their sojourn in Mitzrayim.
This is why we give the bracha to our children that they should be like Efraim and Menashe. We daven that Hashem should infuse our children with that hidden spiritual light that Yosef had, which shone through his Egyptian facade, through his children, and through the Jewish people as a whole. We ask Hashem that our children find that hidden spiritual capacity that may at first be hidden, but has the potential to illuminate their own individual lives, as well as the lives of the entire Jewish people. "May Hashem make you like Efraim and Menashe... may they be like fish, multiplying greatly in the land."