I have 0 nitrates/nitrites after 4 mnths. I think I will leave it in..way too much hassle to remove it and I may just do more harm than good
Having 0 nitrates after only 4 months is not that uncommon, and IMO
not a good indicator to the efficiency of the setup. Crushed coral is not recommended for plenums or any other deep substrate filtration. It will allow too much oxygenated water into the bottom layer of your estimated 2.5" of CC
. Thus, I doubt you will get the benefits of having an anaerobic zone.
Crushed coral also traps a TON of detritus and should be vacuumed regularly. This will disrupt any functionality of your plenum anyway, or if you don't vacuum will just build up with detritus. IMO
, you will be doing far more harm than good in the long run by keeping it in. I would either build a proper plenum or remove it and just have the substrate.