They call it hydroton and this is the cheapest I could find lol...it's clay pebbles usually more round and reddish than what I use.
Once I figured out how deep to flood them and how often, it was off to the races. First grow I was worried about the rez alot and nutrients was a big change too. I now use the same rez to feed my soil grows too......so Jacks 321 for all my grows from start to finish. No finisher products or boosters...I thought sure all that N in flower would result in subpar buds......I was wrong.

I still add fulvic acid, calmag, silica and kelp every once in awhile with hydroguard.
Now I just stick seedlings in at full nutrient levels and they still just take off......it is truly amazing and I highly recommend it.
Better have a BIG room for 12 pots...the growth is crazy.....I mean these are auto's I'm growing in this thing finishing in 70 days or less with good yield. I would never have believed it..