I am sorry but trying to guess what is wrong with your tank without knowing the history of the tank, the corals, which corals are affected , which corals are not affected, how long you have had the corals, how long the tank has been set up, how long the browning has been happening, were the corals healthy at one time, have you made any changes to the tank or your husbrandry is not possible.
It is kind of like you saying you have a tummy ache and need a diagnosis. And a us saying, its a tumor, it indigestion, its your gall bladder, its the food you ate last night, its the full moon. One of us might be right but I wouldn't make drastic changes to my lifestyle without knowing what is truly going on.
All we really know about your tank is some/all of your corals are browning (only a symptom) and your tank parameters are within an acceptable range and you have adequate flow and adequate lighting.
There is a big difference between corals browning and dying. Browning is often caused by an excess growth of zooxanthellae algae. Bleaching/ dying is often caused by the coral expelling the zooxanthellae algae.
Also it is rarely just one thing it is likely a combination of things that are showing up in your corals as browning.
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