Ideally we would all feed 100% organic fish food, at least I believe that to be true. But, is it economically feasible? If cat food keeps your fish healthy, and produces enough manure to feed your plants, give it a shot. Is it ideal? No. But then again, neither is Purina, or any other fish food out there. I would suggest though that you are what you eat, and even though there are cat foods out there designed for cats that are organic, that surely does not mean that they are healthy for fish. I think a healthy fish might be more important than an organic fish?