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I'm thinking the same thing CaptainAhab is thinking... curious if you took ammonia readings AFTER you added the rock. Just because it looked OK, or was sold as "cured", doesn't mean it really was. You might've started another mini cycle with the addition of the rock.
Nitrates, microbubbles from the skimmer, or FW food shouldn't have been an issue. Most likely, you either got a bad fish from the LFS to start with, or your tank is cycling again. If you're buying fish from a large chain store, you might seriously consider the benefits of a quarantine tank before introducing the fish to your display tank.
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