I would try a finer media, my suspicion is that the flow is channeling through the coarser media and not giving a good reaction. You are correct, the effluent should be above 18 dKH for sure and usually 20-24 is what I would adjust to.
I will give Tunze media a try, though the supplier is out of stock... I will report back once I am able to get my hands on it.
What would be the recommended pH inside the reactor to achieve these effluent Alk levels?
Thanks