Thought I would post the whole post as its interesting. From another site... :
"CLEAN....unpainted lava rock has been known to be used as a substitute...sourced from a nursery. The drawbacks here, I'm told, are that there can be contaminents hiding in the pores of the rocks... and that contrary to hydroton which is round, the abrasiveness of the lava rack can aslo be a concern/can be hard on the finer roots.
pea gravel. Hard to keep a clean res/keep things from clogging up.
ornamental river rock...tumbled rock....marbles... I even taught a hydro class to thrid graders here where we set up a bubbler and an ebb/flow and used LEGOS as the growing medium to show em that in true hydro the medium is ONLY a place for the roots to grow into/around for airspace....nothing more.
I would caution using peat/Light Warrior/dirt...etc....anything that HOLDS moisture can easily lead to fungus/root rot/etc and will likely stay too wet. The REAL secret of the DWC or drip in these buckets (IMO) is being able to MOVE the solution...either via bubbles...or via drip and the water running down the root column.
Either way...the exchange of air/water in both instances happens time and time again within the system. In soils/heavier mixes where you cannot water in "cycles" due to probable overwatering, you lose ALL of these benefits, IMO.
LOTS of people grew in CLEAN rocks before hydroton. CLEAN being the key word. IMO. Also avoid LIMESTONE and some other rock is just too damn HEAVY.
hydroton is REUSABLE..so the outlay of cash is a one-time gig. should be about $35 for 50 liter bag."