Quick help!

One of my firefish is hidiing and breathing heavily. My scarlet skunk cleaner shrimp, Royal Gramma, and other firefish are fine. Just this one firefish. I'm scared it might be dying. Please help if you have any info. I tested my water and it lookes fine. Nitrate 0, Ammonia 0, Nitrite , pH 7.8-7.9.... I don't know what to do. The temp is 82. Gravity of salt 1.023.
 
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