That's funny, but it has some truth to it. Most freshwater fish are pretty resilient when it comes to their nutrient needs, unless they are under stress - then not so much. But marine fish, quite the opposite. Not that many years ago many marine species that are now commonly kept in captivity, and eating commercial food, were considered doomed in captivity.
I wrote the following several yrs ago.....
Kieron Dodds, from Tropical Fish Hobbyist magazine wrote an article on the Moorish Idol in 2008, titled; "Still Impossible After All These Years - Keeping Moorish Idol". He clearly admits that the main intent of his article was to discourage anyone from acquiring this species, as at the time he felt this species has almost no chance in being kept alive in captivity beyond a very short duration.(as in a few weeks/months) At one point in the article he stated "Pablo Tepoot is perhaps the single individual who has had the most success with this species" Today, Pablo has tanks full of this species, all thriving for years.
Something that most people would have considered impossible 15 or 20 yrs ago.
Anyone can keep a Moorish Idol or Achilles Tang alive for a brief period of time, but very few manage to keep these fish alive & thriving for years. In the past, many hobbyists would treat these fish with a cut flower mentality, and simply replace them with a new fish when they withered away. Today, there are many hobbyists world-wide that have found that IF their fish are provided with an optimum diet, they too can keep these "doomed in captivity" species, for years.
This hobby has come along ways over the past 50 yrs, and I personally welcome anything that can improve someone's fishkeeping experience. Today we have a multitude of filtration methods & equipment that is offered the hobbyist, controller units that can monitor & control everything from the tanks temperature, lighting (with dawn/dusk/moonlight options), pH, etc., and high quality aquariums that come in all shapes & sizes. Even the species of fish now available to the average hobbyist is something that many fish keepers only dreamed about 50 yrs ago. The science of fish nutrition has also vastly improved from what we knew 50 yrs ago, and many myths & misconceptions from the past, have now been proven to be false.
It's also becoming more clear with each passing study that many of the health disorders that we see in both freshwater and marine fish are directly related to an improper diet. While certain foods may in fact keep a fish that's kept in a totally non-stress environment, healthy, place that same fish under any type of stress & that same food may fail miserably. The problem lies in the fact that when a fish does become ill, most hobbyists aren't going to be able to pin-point the exact cause, especially if it's nutrition related.