Cure the rock in a separate tank. When it is cured and free of all dead stuff and detritus, put it into your display tank with all new water, no water from the curing container. That way, you leave all the detritus in the curing container as well as all the nutrients the curing process created.
FWIW, my page on curing live rock....
http://www.lostmymarblz.com/reeftips-curingliverock.htm