The rock used in marine tanks is generally broken bits from coral reefs. It is very porous which allows water to flow through through it. Being porous also gives it tremendous surface area. Bacteria live on the surface area of the rock. The brown rocks used in fw
tanks do not have any where near the suirface area of the rock used for LR
. It therefore has little benefit and could, depending on its makeup (heavy metals) be detrimental.