I use 3 liters of eheim substrate pro, spherical glass balls about the size of peas, in a mesh bag in my 120's 40 breeder sump.If you do not go anaerobic - no denitrification - no real use of the stuff. But if you put a mesh bag in the sump and not "shake" it every day - probably the inner part will be anaerobic - but then - the Si problem. However if you do regular WC you will limit the concentration - as I did the first months
Sincerely Lasse
I only shake it maybe every 2 months if that.
My no3 has been 1-2 since startup just about a year ago.
I run a no scheduled water change system and only replace skimmer loss and to adjust SG.
Sending out for the first ICP test in 2 weeks when tank turns 1.
Results will be interesting.