Fish Have Feelings, Too: The Inner Lives Of Our 'Underwater Cousins'I have heard that on one occasion the Blessed One was staying near Sāvatthī at Jeta's Grove, Anāthapiṇḍika's monastery. And on that occasion, a large number of boys on the road between Sāvatthī & Jeta's Grove were catching fish. Then early in the morning the Blessed One adjusted his under robe and — carrying his bowl & robes — went into Sāvatthī for alms. He saw the large number of boys on the road between Sāvatthī & Jeta's Grove catching little fish. Seeing them, he went up to them and, on arrival, said to them, "Boys, do you fear pain? Do you dislike pain?"
"Yes, lord, we fear pain. We dislike pain."
Then, on realizing the significance of that, the Blessed One on that occasion exclaimed:
If you fear pain,
if you dislike pain,
don't anywhere do an evil deed
in open or in secret.
If you're doing or will do
an evil deed,
you won't escape pain
as you run away.
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Fish Could Be Conscious
Fish Are Sentient and Emotional Beings and Clearly Feel Pain: Fish Deserve Better Treatment Based on Data on Their Emotional Lives
We should neither kill nor harm fish, or even insects, even if you aren't buddhist or following any dharmic religion:
What insects can tell us about the origins of consciousness
And yes, you should do your personal best to go vegan, even if you don't rack up negative kamma for purchasing previously slaughtered animals at the market.