The cycle needs an ammonia source. That means putting in a shrimp and letting it decompose or dosing with ammonia. The die-off from the live rock, if there was any, is not enough. Were you testing your water and did you see the ammonia rise, followed by nitrites, then a bit of all three and then both ammonia and nitrites ending back at zero with high nitrates? If not, your tank did not cycle. All three should not be zero at the end. That is what regular saltwater bought from the LFS
What are your parameters now? I'm guessing you added fish, your cycle started and you now have ammonia and nitrites, which burn the gils and cause heavy breathing and/or death.
Putting the fish in QT now has it back in an uncycled tank and the ammonia from its waste will start that cycling.