Only time I let my cats (and my dog) eat the fish food is if they get at it themselves and devour it without my noticing. The flake food I didn't mind so much, since I prefer pellets and real food and the flakes came with a little tank I got for a Betta. It drives me nuts, though, when I'm feeding pellets and wafers and Buffy (one of the cats) hops into the tank's cupboard and starts ripping away at the pellet packets. Thanks for helping to make the food readily available, Buffy dear, but I like the fact that they can be sealed for quality freshness. With gigantic holes in the bags, the ziplocks don't really serve a purpose anymore. Eh, but if she's gotten at a bag, I just put the pellets or wafers or whatever into a little container, and then she can't get to it at all. Not that she stops trying. And if she knocks a container to the floor, it becomes available to my dog, and he's a master at breaking open containers and pop bottles and anything like that. I allow him to destroy those kinds of things, since it works his mental stimulation, but it's a bit of a bummer to be looking for a container of food and finally find the crushed, dog-chewed plastic buried in his crate, food and lid nowhere to be seen.
Actually, I would think cat food (if it were fish-based) would be healthier for a fish than fishfood is for a cat. That is, if you're using premium-quality cat food, like California Natural or Innova (not Whiskas or Purina, ugh). The best fish food I've found has more fillers than the worst cat food I allow into my house. It's got me wondering, now, though. What would the fish do if I were to crack out a kibble and crush it, and sink it in a tank? I don't feed kibbles anymore, though, so I gotta keep wondering unless someone else online tries it, lol
I think (and hope!) it goes without saying that no animal should ever be fed, as a major or even medium part of their daily diet, anything that's not specially designed for their species.