Latest update.
After I dumped most of them, I still left a 2inch layer under my current 4ft. So far I have undetectable nitrates despite having a sandbed of around 3inchs only.
I conclude that the dense glassbeads to have powerful denitrification. When they were removed, hydrogen sulphite were released (rotton egg smell). For those that wondered why I had undetectable nitrates despite of no skimmer, no refuge ... this is my secret!
