Copied from the FAQ sticky:
If it becomes necessary to treat your fish while they are in QT, then porous materials, such as sand, rocks and bleached corals can become an impediment. Live rock, sand, crushed coral, dead coral, etc. will absorb some meds that you may need to use to treat diseases making it extremely difficult to maintain a therapeutic dosage of the med during treatment. You want to create a sterile environment that minimizes fish stress and is conducive to fish treatment if it becomes necessary to deal with disease. If, however, you want to use some rock rubble, you can, just know that if the need to medicate your QT/fish comes up, then the rocks should come out [and not be returned to your main tank!]
:happyfish