Just a brief look at them, and they seem to be more of a chemical filter than a bio-filter.....anyone please educate me here too.
Why not go with some base rock and add a little LR
Rock does more than hosts the bio-filter, like dividing the tank for territory and providing hiding spaces as well as sleeping quarters...
I guess there is bio-filtration in there to, I found this:
It seems mostly to reference FW