Some Brahmins are known to be lovers and avid eaters of fish.
Are Brahmins allowed to eat fish?
Yes, eating fish is against the regulations for Dvijas. Manusmriti verse 5.15 reads:
yo yasya māṃsamaśnāti sa tanmāṃsāda ucyate | matsyādaḥ sarvamāṃsādastasmān matsyān vivarjayet || 15 ||
He who eats the flesh of an animal, is called the ‘eater of its flesh’; he who eats fish is the ‘eater of all kinds of flesh’; hence one shall avoid fish.—(15).
So, eating Fish is equivalent to eating all kinds of meat; hence, one shall avoid fish.
Eating forbidden food is equivalent to drinking alcohol, which is a Mahapataka for a Dvija. Manusmriti 11.56 reads:
brahmojjhatā vedanindā kauṭasākṣyaṃ suhṛdvadhaḥ | garhitānādyayorjagdhiḥ surāpānasamāni ṣaṭ || 56 ||
Neglecting the Veda, reviling the Veda, bearing false witness, slaying a friend, and eating of forbidden and unfit food,—these six are equal to ‘wine [alcohol made of grains] -drinking.’—(56)
However, certain fish can be eaten without having offered them in a sacrifice. Manusmriti verse 5.16:
The ‘Pāṭhīna’ and the ‘Rohita’ are fit to be eaten when used as offerings to gods or Pitṛs; the ‘Rājīva’, the ‘Siṃhatuṇḍa’ and the ‘Saśalka,’ (one may eaṭ) on all occasions.—(16)
Yājñavalkya verses 1.177-178:
Among fish the following may be eaten by the twice-born,—Siṃhatuṇḍa, Rohita, Pāṭhīna and those with scales.
However, many commentaries state that even these fish should be eaten only after offering them in a yajna (sacrifice).