Fantastic. IMO this is the only way to go. Harvesting rock from the sea is a major problem, and if it's not done in an eco friendly process it shouldn't be done. Think of clear cutting a forest, you're not just taking the tree's out of environment...anyway before I get carried away I'll get back to the topic.
After your tank has "cycled" the rocks they are basically live. Nitrogen cycle is roughly a month, as I'm sure you know. If you're adding into the sump of an established system that has already cycled it would be safest to wait at least a month for the nitrifying bacteria to grow on the rocks. Good indications of the rocks being live and ready for use is the diatom bloom. When dead rocks become live they usually go through a diatom bloom after 2-3 weeks. Once those diatoms go away wait a couple weeks and the rock is good to go IME.
If you want life rich rocks then Salty has it right....being proactive with dead rock is key for rich dead rocks. I made my nano using 20 lbs of dead DIY oyster shell and cement mix rocks with 2 lbs of really nice 2 year old live rock that was once dead rock also, and came from a very healthy pest free tank. It takes time but there's things you can do to speed up the process.
1) place an already live rock in a good flow area. Right in front of a power head or overflow return that blows onto the dead rock will help seed the stuff from the live rock.
2) Ask local reefers that have good healthy tanks for a small cup of sand, or some of the water from their water changes to diversify the critters and bacteria in/on the rocks. Or add from your own tanks.
3) turn the flow off and scrape coraline from the live rocks so that it floats around and lands on your dead rocks. The bits that attach will grow.
4) good water quality as always.
Now the bennefits are...for those wondering....
1) cool shaped rocks that are fun to aquascape with.
2) very slim chance of introducing pest organisms like aptasia, hydroids, mantis shrimp, bubble algae, etc. I even gone as far as re-fragging my corals when I get them, and inspecting them under bright flashlight and a magifying glass. ;P No pests in my system
3) save the natural reefs
4) cheap ($0.20 a lb for my rocks
5) really amazing to watch dead rocks come to life.
6) feel good about yourself and the investment.
&) so many more reasons.
cheers
