there are so many thing that could have been on that rock when it "died" that you do not want anywhere near your tank. Using bleach will help to rid any left overs that could be harsh on your tank, also taking a brush and scrubbing it is a must along with blasting the crap out of it with a hose.
Ok here is what I did.
i lay all rock in my driveway and blast with hose for a long time
put rock in in container with bleach water, mine was a 20 tub with one full big bottle of bleach and the rest water.
made sure I had power heads in tub to circulate water. did this for about 5 days to a week.
took rocks out and lay in driveway again and blast with water. FOR A LONG TIME.
Put rocks back in tub but this time with only ro water and power heads. Let sit for a week.
take rock put in driveway and blast with water. for a long time.
Put rocks back in tub with ro and power heads, let sit for another week.
by this time I was ready to put into my new tank, I took rocks out of water and put them into the new tank with a few pieces of live rock to seed.
Waited a few months and then adding the first coral.
I would always stay on the safe side when it comes to setting up your tank. Doing it right from the start can save you lots o monies : )