All bodies use different trace elements, ions, and minerals to regulate hormone development, osmotic processes, amino acid development and use, tissue development, scale development, pigment cell development, etc. Basically, every physiological process in the body requires necessary elements to function properly.
Fish have evolved to take in needed trace elements differently then air-breathing humans. Many elements are absorbed and properly routed through gill tissues and palates.
i have to believe that almost all, if not all, the necessary trace minerals can come from a fish's diet. the same way freshwater fish, or brackish fish get their trace elements while living in fresh water.
i don't believe that marine aquarium water needs to be so complicated because i have heard of people keeping fish in synthetic ocean water made of just a few (3 or 4) elements. i believe it might be possible to narrow down that already thin list.