Lava rock is generally inert, and I use it as bio-media in my sumps, for my planted refugiums I like pool filter sand as substrate, which is usually inert too.
Debris doesn't work its way into PFS, just sits on top, snd makes maintenance easy, as I can easily vacuum it up.
I like to place lava rock or any other bio-media in mesh bags, in order for ease of maintenance too. Simple to pull the bags out and shake around in a bucket of water change water, and when rinsed toss the bags back in the sump.
Planted refugium below with aquatic plants, and PFS substrate.
some of my refugiums use emergent plants, like papyrus.