Humans and Dogs are omnivores yet research has proven both could live healthy on VEGAN diets. Cats are carnivores(!) And with them as well research has shown that they can live on a VEGAN diet(with Taurine supplment).
Looking at the literature, most of it is self reported, and could be down to other factors (Generally vegan dog owners may be as a demographic more likely to tend to their dog's other needs par example)
On vegan fish food, Nada, to my knowledge, nobody has bothered catering to an ethical consideration few actually have, instead generally most feeds have some sort of animal protein, even stuff like algae wafers! Just get a herbivorous fish and feed them plant matter. I will say, in a functioning tank, there is generally a miofauna population that will inevitably be consumed by a fish at some point, this is to say that try as you might Some animals are always eaten.
The border between herbivore and omnivore in basically any ecosystem is really shaky, and mostly down to "can I eat that thing without expending much effort" (just look at deer munching on baby birds. )