Ok...this is an interesting conversation...I shall throw my two cents in.
I 100% agree with what has been said up to this point. Fish cannot be fed a vegan diet when they are carnivores or omnivores. Come up with as much evidence supporting your case as you like, but it's just fact. If the fish are carnivores or omnivores, they will perish under a vegan diet. This is the same with cats and dogs. Can they handle a diet like this? Yes, but they will die sooner. You can look up many reputable articles that prove that. If you try to change nature, it won't work because nature is nature. Humans and animals were created for specific diets. If humans and animals go off course of those diets, there will be consequences no matter how "wrong" or "immoral" it feels to eat these diets.
Humans are omnivores, and we are hardy creatures. We can eat a lot of different things and be perfectly fine. If you pursue a vegan diet, that's your choice. But in time, since we are omnivores and we are made to eat meat as well as greens, we will become unhealthy as we will lack the nutrients that meat provides on a vegan diet (and it can't be supplemented by pills). Sure, vegan diets can help us lose weight, among other benefits, but after a while, it will become harmful for us in as many ways as there are benefits.
So, to answer the OP's question, what you are looking for is harmful to the fish. Stop trying to force nature to comply with your moral standards. Your dogs, cats, and fish don't care about your moral standards; only you care about them. They simply want to eat what they were created to eat, not be forced to eat a diet that is inappropriate for them.
One last thing: you say that you are "rescuing" these animals, but what you say is not what you practice. Maybe they did come from bad homes where they were poorly taken care of, but you aren't doing them any favors when you take them from one bad home and put them in your home where you feed them inappropriate food and consider feeding them inappropriate food because of your moral standards. They might as well be left in the home they were in originally.
I hope you read all of these people's comments and seriously soak in what they are saying.