Is there a Chiuv to eat fish at all three meals on Shabbos?
Halachically Speaking Vol. 3 Issue 9 (Avoiding Danger) brings many reasons why fish is eaten on Shabbos (footnote 45) - Some of these were brought in other answers:
Some say just like fish have their eyes opened, Hashem has his eyes opened at those who fear him and have compassion on us (Otzer Hayidios Shabbos page 82:11).
Water is the source of life for fish, and Torah is the source of life for Klal Yisroel which is comparable to water eating fish reminds us that we should devote Shabbos for learning of the Torah (Sefer Matamim page 28:2 new).
The daled and gimdel of dagim represent the three forefathers and the four emuhos.
Also the mon is called gud which did not fall on Shabbos and are the letters of Dag (Fish) (Sefer Matamim page 28:7 new).
Hashem blessed three things at their creation. The fish on the fifth day, man on the sixth day, and Shabbos. When people eat fish on Shabbos it brings together these three blessings together in “a three-stranded” cord that does not break easily (Sefer Matamim page 27:1 (new) Bnei Yissochor Shabbos 3:16:pages 11b-12 new).
Noach’s descendants were not allowed to eat fish until the Torah was given to Klal Yisroel on Shabbos (Hamodia Magazine 26 Shevat page 10).
Fish never sleep. This reminds us to utilize every moment of Shabbos and not to waste time by sleeping excessively (Hamodia Magazine 26 Shevat page 10).
The gematria of dagim is zayin nun spelling zayin, alluding to the seventh day of the week (Hamodia Magazine 26 Shevat page 10).
Another reason is brought here:
- Another reason that people know for eating fish on Shabbos is based on the Megaleh Amukos in Vasechanan, based on the Zohar, that the Neshomos of Tzaddikim that have to return to this world for a tikun, come as fish because fish can be eaten without being slaughtered, a procedure through which they could become a nevaila. The Zohar says it is because the Chachomim are referred to as “Asifa”
- Wherewith does one show his delight therein? — Rab Judah son of R. Samuel b. Shilath said in Rab's name: With a dish of beets, large fish, and heads of garlic. R. Hiyya b. Ashi said in Rab's name: Even a trifle, if it is prepared in honor of the Sabbath, is delight. What is it [the trifle]? — Said R. Papa: A pie of fish-hash. (translated here as "fish fried in its own oil with flour")
- Others suggest that eating fish on Shabbat represents the feast which will take place in Olam Haba, the next world, in which the Leviathan fish will be served.
I heard this reason, but have not found a source for it:
מי שאוכל דג ביום דג,ניצל מדג.
This means to say: One who eats fish (דג) on the 7th day (ד=4,ג=3 3+4=7) Will be saved from the Judgments of Gehenom (ד"ג שהם דיני גיהינום ).
None of this actually answers if we have to eat fish for all 3 meals.
The Kitzur Shulchan Aruch (72:7) says that it is a mitzva to eat fish at every meal. (apparently quoting the Mogen Avraham Orech Chaim 242:1), unless it will adversely affects one's health or one doesn't like fish, for the Sabbath was given for our pleasure, and not for our discomfort. (English translation here).
This memra comes from Tikunei Shabbos. The M.B. 242:2 adds on the word "tov" (not a very strong language). The B'er Hetiv there says each person according to his nature. The idea is you should have oneg shabbos and fish was considered a nice dish. If you don't have the appetite, don't eat it (next phrase in the M.B there).
I have seen it brought down in the name of the Belzer Rebbe (looking for the citation) that the three things that immediately follow the word ויברך in ma'ase b'reshis are humans, shabas, and fish. Therefore the intersection of all three (humans eating fish on shabas) is a particularly strong conduit of b'racha. No mention of a chiyuv.
From Shulchan Aruch Harav, quoting the words of the Alter Rebbe: It is desirable that even a person whose financial resources do not suffice for him to indulge in many Shabbos foods be meticulous in not having fewer than two types of cooked food [for his Shabbos meals]. It is desirable to be meticulous and partake of fish during every meal, unless [a person’s physical constitution is such that eating] fish is harmful to him or he dislikes them, i.e., it brings him discomfort, not pleasure — for Shabbos was given for the sake of pleasure... (Hilchos Shabbos - section 242 - To Be Meticulous in Honoring the Shabbos - halachah 7)
Here is the reason why you eat it with your Hands: