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do you have a sump? how much lr do you have? what kind of skimmer? dsb? cheato? need some system details to help you...
4 t5's 18" under water is not to much light for a sps coral...check it with a par meter.
i would lay money on the low alk and the no3 being your problem, do a major water change and get your no3 to as close to zero as possible.
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