I agree also as your tank borders nutrient poor you may notice that they may not fair as well as they once did . Example you set up a new tank ....Once everything is running you may start to add things most times people think of zoos because of their known adaptability and they are most times very hardy . They thrive in the new setup for a good amount of time but as the tank fully matures things take a different route and we can starve our tank of nutrients . That inturn will cause zoo's and other corals to not do so well in the low nutrient enviroment unless there is food provided to the corals by means of feeding more to your fish but not to the point you are affecting water quality , feeding your corals with foods like rotifers,oyster eggs ect. As long as a good food source is provided low nutrients (No2,No3,Po4)shouldn't be a problem .