I agree with 60 reefer here. It is most likely nutrient deprivation and the corals are slowly dying off. They may not show signs for month and then all of a sudden they go downhill fast. Some corals may be more resilient then others or use more light for nourishment then actual food substance. Your ICP will help but you need to get nitrates to 10ppm and phosphate to .05 to .1 for an LPS tank.