Re: "vegan fish"
Mine happily eat sweetcorn, peas, crisps, milk, cheese and pretty much anything else they can steal.
Yeah, but they can't digest it properly. My cat's a complete bastard for crisps (can't open a packet without him trying to intercept them somewhere between fingers and gob) but he doesn't get any real benefit except for the fact that he likes the taste (and some of the salts will make their way into his system).
As the CrazyCatDude said, felines are obligate carnivores and won't survive on a vegetable diet unless it contains specific additives.