Re: "vegan fish"
A vegan once fed their cat on a vegan diet. Poor thing almost died
Hardly surprising - cats are pure carnivores. Amongst other things, they don't have the ability to create taurine (since their usual prey contains large amounts, this isn't usually a problem).
So, unless the vegan food contained all the nutrients that they need (unlikely, unless it's specifically formulated for cats) then they will quite quickly start to die of taurine deficiency.
("Taurine is essential for cardiovascular function, and development and function of skeletal muscle, the retina, and the central nervous system". Fine for us (and dogs, and pigs - omnivores in general) because our bodies can synthesise it. Not so much for cats.)