the tank has been up for more than 6 months. I have a 65 gallon with yellow tang,6 line wrasse, true percula clown, and a cardinal. Only new coral I have added were a couple Aussie acans. Water is perfect according to my LFS. No nitrites, nitrates at 15. Ph8.4 no phosphates. It doesnt look like something killed them. Could it be lack of food?