Sometime to add on. Apart from using NO3 test kit to understand if the denitrification has established, one can also make use of an ORP meter. If ORP is in the range of -50mv to -250mv. It shows the presences of denitrification bacteria. if the ORP reading drops below -250mv, then the bacteria inside the reaction chamber is actually producing H2S. As such need to increase the drip rate to push orp again.
This pcs of information, I got from the aquamedic 400L denitrator manual.