Live rock cooking help

[makana]

Ro/di water. It has been a while but i have read Myaka's guide along with most of the guides out there. For some reason I was expecting that the end of the detrius being pushed out of the rock would be the end of the cooking. I was surprised to see the nitrate issue. As far as I know live rock can't store or leech nitrate so I don't really understand where the nitrate is coming from. Its nice to know it is normal but it would help if I had some idea of where the nitrates are coming from in this situation.