I use 23 liters of siporax in my sump, not like forced flow. I like to think little passive water flow it will have less oxygen in it facilitating the formation of anaerobic bacterial cultures inside and not force debris into your pores.
I know that the amount is much higher than the SERA suggests, but I also know that it can do no harm
that rock in the first part of the sump was just to acclimate a fish, is not part of filtering