Eating Fish on Shabbos:
Eating fish is included in the Mitzvah of Oneg Shabbos. In the times of the Talmud Oneg Shabbos was fulfilled through eating large fish. Fish should be eaten in every meal, unless it is hazardous for his health, or he despises eating fish to the point that he does not receive pleasure in eating it but rather pain. It should especially be eaten by the third meal.
 Admur 242:7
 Admur 242:2; Rav Yehuda in Shabbos 118a
 Admur ibid; M”A 242:1 in name of Tikkunei Shabbos; M”B 242:2
 Admur ibid; M”A ibid
The reason: In such a case he should not eat fish, as Shabbos was given for pleasure. [ibid]
 Siddur. Sefer Chareidim [chapter 33] states it is a mitzvah to eat fish by all the meals, especially by third meal in order to elevate the souls that have been reincarnated into the fish. In the writings of the Arizal it is taught that the souls of the Tzadikim are reincarnated into fish. [See Piskeiy Teshuvos 242 footnote 63] In Kuntrus Achron 242:4 Admur mentions an opinion which rules that eating fish on Shabbos is a Biblical command. However, Admur rejects this ruling saying there is no legal basis to say that the Sages instituted specifically fish to be eaten.