Datnoides, are difficult to get onto processed or frozen foods, the process of getting them to quit live is a patient one.
These fish will starve to a point of no return, and once there they simply wither to death.
In my experience Ntt and Ngt are very stubborn and have seen em starve to a point, after which they are so wasted that even if live food is offered they don't have it in them to catch and eat.
The best way is to not starve them for more than two weeks. Try feeding them Pellets every third day.
In your case the fish was a new entrant, I always feed new fish, what they would eat, and slowly get them onto other favorable foods.