May one eat fish by the large meal eaten prior to the Seudas Hamafsekes??
Yes it is permitted to eat fish during the accustomed large meal that takes place before Mincha. The restriction against eating fish only applies to the Seudas Hamafsekes meal which is the last meal eaten before Tisha B’av, after midday, and even then its restriction is due to custom and is under debate, [although we are stringent by this last meal].
Sources: See Michaber 552:2 “and fish”; Beis Yosef 522; M”A 552:2; Hagahos Ashri 4:35; Mamar Mordechai 552:2; Machazik Bracha 552:4; Kaf Hachaim 552:17-18; Poskim who permit even by Seudas Hamafsekes: Nachals Tzevi 552, brought and negated in M”A ibid