It was kept in captivity by you though, which is not it's natural habitat so any choices it made were, arguably, under duress.

If you lived by a creek and regularly recognised a fish swimming by, and one day this fish killed itself in front of you- you still shouldn't eat it as fish contain a lot of parasites and there's very likely also something toxic in the water causing the fish to harm itself this way.

But yeah, sure, hypothetically: if for a year or so you knew a wild fish that lived in an unpolluted and ecologically healthy body of water, and one day this fish chose to kill itself in front of you. You could, if you really wanted to eat a suicidal fish, eat the fish and say it was vegan because the only harm that came to the fish was through the un-coerced choices of said suicidal fish.