Dry Pikani bleach than acid bath

[asylumdown]

Remember that these rocks aren't necessarily pure calcium carbonate - there's probably plenty of mineral contaminates in the rock that were either laid down by the corals themselves, or precipitated out through various chemical processes after the corals that built the rock died trapped within them.

A little discolouration at the end of the whole process wouldn't surprise me in the least, and you might need to dissolve the entire rock to truly get it all out.