I have never replaced any of the EhfiSubstrat I know I am suppose to replace 1/3 periodically because the pores get clogged with bacteria and then it doesn't catch as much. I heard some people boil part of the EhfiSubstrat to remove the trapped bacteria in stead of buying new. But I have never tried that. I've cleaned out the whole thing before but will need to do get some new EhfiSubstrat soon.
I'd use 2x this amount of filter.
Good current etc.
Here's my non CO2:
I use a Rena 3(350 gph) and a rio powerhead for current.
Safety backup if anything clogs etc.
Plenty of O2 and adds new CO2(even if limiting and low).
This is better for fish no matter how you slice it.
When we add all this structure and plant biomass, this reduces the circulation a GREAT deal.
Plants are doing good, so that is plus.
But........fish always come first, we can fix plant probleme without losing them or the fish.....but cannot bring fish back from the dead.
So always error of the fish side of things.
Without CO2, there's no good reason not to have good current.
Even with CO2........
Fish like it.
Yes, clean the filter maybe once every 2-3 months, I use a sponge prefilter slipped over the intake,this reduces muck and I clean that weekly.