I was told in my local grow shop that re-useing clay pebbles decrease yeild. I do not know this for sure as I re-used pebbles and made some mistakes on my second grow so really couldn't tell what was to blame, but as a novice will take there word for it and see what happens.. But Cannazym is ment to desolve dead roots and I was told to use that if I couldn't be botherd to rinse out the roots but to be honest I think it best to dump the old lot and start with a fresh rinsed out new lot, doesn't cost a lot and saves on the old back ache and blocking up ur plug hole bla de bla.