There are certain restrictions regarding the foods which may be eaten at the Se’uda Mafseket, the final meal before Tisha B’Ab. The Shulhan Aruch (Orah Haim 552:2) writes that the custom is not to eat "Besar Dagim" ("the meat of fish") during this meal. This is the text of the ruling as it appears in prevalent editions of the Shulhan Aruch. However, the work Nahalat Sevi claimed that this text is erroneous, and that this passage should actually read, "Besar Gedayim" – "the meat of young goats." The Nahalat Sevi notes that if the Shulhan Aruch was speaking about fish, then he would have simply written, "Dagim" ("fish"), and would not have used the unusual term "Besar Dagim" ("the meat of fish"). It is therefore more likely, he claims, that the Shulhan Aruch actually wrote, "Besar Gedayim," and maintained that one should not eat goat meat at the Se’uda Mafseket. According to this view, then, it is permissible to eat fish at this meal. Indeed, in the older edition of the Shulhan Aruch printed in Venice, the text reads, "Besar Gedayim."
However, in his Bet Yosef, Maran (author of the Shulhan Aruch) cites the ruling of the Or Zarua (Rav Yishak of Vienna, 1180-1250) that one should not eat fish at the Se’uda Mafseket. This position is adopted as the final Halacha by a number of authorities, including the Hid"a (Rav Haim Yosef David Azulai, 1724-1806) and Rav Haim Palachi (Turkey, 1788-1869).
Some authorities distinguish between different kinds of fish in this regard, based upon a discussion of the Magen Abraham (Rav Abraham Gombiner, Poland, 1637-1683). The Magen Abraham discusses the question of whether one may eat in the Se’uda Mafseket fish that was fried with a batter made with eggs. Only one cooked food item is allowed at this meal, giving rise to the question of whether fish fried with a batter constitutes a single cooked food or should be considered two separate fried foods, which would be forbidden to eat at this meal. Later authorities noted that since fish is any event forbidden at the Se’uda Mafseket, the question of whether to count this food as one or two cooked items is irrelevant. Evidently, the Magen Abraham held that certain types of fish are permissible at the Se’uda Mafseket. The custom not to eat fish applies only to large fish, which are considered a luxury, but not to small fish such as canned sardines, which are a simple, ordinary food and thus may be eaten at the Se’uda Mafseket. This is, indeed, the ruling of Hacham Ovadia Yosef, that one should not eat large fish at the final meal before Tisha B’A, but it is permissible to eat small fish at this meal.
Summary: It is permissible to eat small fish, like canned sardines, at the final meal before Tisha B’Ab, but one should not eat large fish at this meal.
