Gotta ask for info first, what are your parameters at? Nitrate and phos? Alk
Your Duncans are closing cause they are not happy, do you have a lot of soft coral that could be sending out toxins?
How old is the tank? If it's in the same spot for 5 years I'd assume its been around a while, could your sand bed be going toxic? Old tank syndrome? Are your other corals looking good?
To really help there is so much info you need to give up. First things first, check all parameters, then go from there...