I have a aquarium maintenance serice, and also had problems about Green Chromis dwindling down. After reading many post, i discovered that many had success by frequent feedings. So I got an Eheim feeder and set it up to drop fine food every 2 hours, 4 times a day. I also tainted this food with focus/metroplex because the chromis are prone to uronema. Low and behold, 8 months later, and the nine chromis are all thriving and doing great. Even one that was blind and 2 that had bruises recovered in a week. I used this meathod in several of my service tanks, and it worked there also.
I may be an exception but I put 4 chromis in my tank and 2 years later I have 3! Lost the first one after the 2nd day but the rest have been happy! I do have an auto feeder set for twice a day feedings