The differences between live rock from one store to another are pretty slim if any at all. When your looking at live rock I like (personal opinion) to find rock that is porous and light, the heavier the rock the more you'll need to get the aquascaping done. If you had a brick or a porous rock that weighed the same, in my opinion the porous rock would be better, you'd have a larger peice to work with as well as more area for bacteria and small inverts and other life.
Doug
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