The B’nei Yissaschar, Rav Tzvi Elimelech of Dinov, taught that when HaShem created the world He only blessed three creations. First up is when God blessed the fish; “God blessed them, saying, ‘Be fruitful and multiply, and fill the waters of the seas…” (Breishis 1:22).
The second blessing was bestowed upon mankind; “God blessed them (Adam and Chava). God said to them, ‘Be fruitful and multiply, and fill the earth!”(Breishis 1:28)
The third blessing was conferred on the worlds first Shabbos, with the words, “God blessed the seventh day and He declared it holy…” (Breishis 2:3)
So, says the B’nei Yissaschar, by you (blessing 2) eating fish (blessing 1) on Shabbos (blessing 3) you merit that all three blessings of creation are being actualized and fulfilled through you.
Furthermore, The Arizal wrote in the famous Azamer B’Shvochin sang customarily on Friday night, that fish are a delicacy on shabbos. Basing himself on this, The Baal Shem Tov would serve fish at every Shabbos meal. Especially Shalosh Seudos.