It's one of the best ways to cycle a tank and is usually recommended by aquarist. The die off from the rock is what cycles the tank.
One of the best ways to cycle a tank is through the introduction of live rock. These are natural pieces found in the ocean and contain a multitude of life in and on them to get your cycling underway. Placing live rock into your new system jump starts the cycling process. Ammonia is produced as some animals on the rock die off and others excrete waste. In addition, the live rock also contains the beneficial bacteria groups needed to convert ammonia to nitrites, and nitrites to nitrates.
The time it takes to cycle a tank with live rock varies on how 'fresh' the rock is. Rock recently harvested from the ocean (not 'cured') will have a great deal of life on it that will not survive the rigors of transportation and being removed from the ocean. This rock will have a lot of die-off that will trigger significant ammonia and nitrite spikes which can make the cycling process last a few weeks.
There is also pre-cured live rock, which means that the vast majority of the die-off has already occurred before you receive it, however, further transportation can still cause smaller ammonia and nitrite spikes. Cycling with pre-cured live rock can still take 2–4 weeks or so.
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Then, finally, there is cured live rock. Fully cured rock that is quickly placed into your tank with minimal transportation time, such as from your LFS (local fish store) to your home, may 'instantly' cycle your tank. The beneficial bacteria groups are already there in good numbers with no die-off to trigger ammonia spikes of any significance. With fully cured live rock you may never see the typical cycling progression of ammonia —> nitrite —> nitrate. Your levels may initially read — and remain at — zero, even while you are slowly stocking your tank.
To cycle your tank with live rock, an effective amount to start with is 1 pound or more of rock per 5 gallons of tank volume. If you plan on utilizing live rock as your chief method of filtration, approximately 1 pound of rock per gallon of water is ultimately recommended.
