Chefri,
Everyone wanted a DSB to reduce nitrate, and this normally takes 9-12 mths to mature before it can function, dont bother whether it's live or dead sand, more important of DSB is really deep with grade 0 sand.
You mentioned 12cm which is almost 5". I suggest you to add another 3cm. After a while the deep will be reduced as the sand tighten.
U cannot fully depends on DSB if the load is high, try refugium too while u are waiting for the dsb to mature.