[fkshiu]

Mother Nature designed live rock to last for eons - no one changes out the LR in the ocean, in other words.

The issue in aquariums is that through poor husbandry habits and lack of maintenance your LR can become clogged with detritus and nuisance algae. LR works because its porous nature allows for anaerobic denitrification deep within its structures. With poor flow and enough crap, this denitrification can slow down or cease altogether. This is why proper flow and blasting LR with something like a turkey baster during water changes is recommended. This is also why the technique known as "cooking" LR is sometimes used whereby it is kept in the dark for extended periods of time to slough off algae and detritus is sometimes used to "re-charge" live rock.

You might want to check out PaulB's reef which is featured in this month's Reefkeeping Magazine online. His reef has been running continuously since the 70s with the same reverse flow undergravel filter and much of the same rock (both collected and manufactured).