Hmm, I hate it when its zero so early on. You either got really good/cured rock with no die off or you haven't started cycling yet. I would recommend waiting a little longer and testing a bit more especially since you haven't added your sand yet. A CUC will probably not survive an ammonia spike if you have one.
Edit: You should ask your LFS about the quality of the rock you got and possibly bring them a water sample to double check yours. Knowing whether or not your rock was fully cured when you bought it would be helpful. Just keep in mind LFS's are businesses and therefore must make a profit. Not saying they would intentionally give bad advice but I have been to a few that do.