I wholeheartedly agree with the water argument, but I digress....
Having studied and taught clinical and therapeutic nutrition (both animal and human), I believe that there is much we don't know about the nutrition of non-domesticated species and this often boils down to us feeding an inadequate nutritional profile to our animals. For the animal, this often results in a shorter lifespan and occasionally nutritional deficiencies that can be hard to detect. For example, take the common budgie, in captivity on an all-seed diet, these birds live around 4 years but if fed a varied diet (in an effort to simulate a nutritional profile similar to that achieved by natural variance) they can live 20 years. It is this significant difference in lifespan that has prompted greater research into avian nutrition. Now it is known that vitamin deficiencies result in poor skin health, decreased fertility, respiratory problems, reduced resistance to infection and parasites, vision problems, lack of appetite...I could go on an on but my point is, if your fish came down with any of those problems, would you associate it with nutrition? Probably not, which is exactly the reason why I try to feed as varied a diet as I can.