How large is your tank. Are you dosing the tank with anything? Did you match the salinity and temp of your PWC
water to exactly the same as the water in your main tank? Any other fish in the tank? What are your tank parameters. And please, spare us the, "water conditions are just fine" routine. You need to put down exactly what your tests kits read. Nitrite, Nitrate, Ammonia, pH, Salinity, and Calcium, Carbonate Hardness, and alkalinity if you do have tests for those. But for now, Nitrite, Nitrate, Ammonia, pH, and Salinity will give us a better idea of what is going on inside your tank. Better safe than sorry, and if your water tests ARE all in fact "fine" and the water is not the problem.. then we can start to think about the history of the tank and what has been going on so far. And i reiterate.. how large is your tank?