I like used rock the best. It's good to know what you're looking for though to know not to get rock that has pests like aiptasia, majano, flatworms, hydroids, etc or nuisance algaes like bryopsis, hair algae, padina, valonia, lobophora, dictyota, sargassum, etc on it. If the rock is pest/nuisance algae free then I think it is the best option. You KNOW what you're getting as far as the pieces go, as well as what is growing on it.
Second choice would be online vendor. I would choose to "cook" the rock for several weeks in the dark to cure it before using it to cycle the tank. This is to try to kill nuisance algaes that are likely to be present on the rock even if they aren't yet visible.
It would be a very desparate measure for me to buy rock from the LFS.
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~ Mindy
SPS fanatic.
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