once elephantnoses get sunken bodies from starvation they invariably die. It is the most common cause of death in the species in aquariums (they often arrive in the shipments already emaciated), and you generally can't save them even with more food.
In this case it may well be old age - these fish have been at the zoo as long as I have been visiting and many are looking a bit doddery - I don't think any new ones have been brought in in years.
