First of all
1) Most of the Bengalis, does not matter whether he or she is Brahmin or not ( in traditional religious sense) does not believe in caste and largely they are non religious. Its a result of long struggle by many social reformer like Vidyasagr, Ram Mohan, and religious leader like Chaitanya and Vivekananda.
2) Fish is one of the main component of Bengali food. They eat fish in every main meal (Lunch, Dinner..) and everyday. And as Fish is not sacred to them (in religious sense) rather just s one type of food..so Brahmin also eats fish and also meat. Also I know many people (Brahmin) who also eat Beef. By the if you believe in Religion and Veda...remember in Rigveda is was written people use to serve veal meet to guest. Beef was not prohibited during Vedic period. Remember Ramakrishna Paramhangsa and Vivekananda also used to eat fish and meat.
3) Fish is one of the main part of tradition. When Bengalis get married..Rohu fish was sent to brides and grooms house by each other and a marriage can not properly finished until the Fish arrive at home.
4) There are another set of Brahmin in India who also eat fish …they are Maithili in Bihar. Bengalis are originated from Maithilis ..Maithili language is very close to Bengali language…and Maithili and Bengali are share many common customs…(like role of fish during marriage) .