I had a very bad experience with red lava rocks, i used them for my first grow with local soil, i made a bed on the bottom to assist with drainage and everything was ok, until the roots touched the lava rocks at the bottom, i know this now. Unless it has been treated and rinsed/buffered and all, do not use them straight out of the bag. I made that mistake too.
Check it out, it was @Herbie, @Uncle_Al and @ryasco who saved that harvest for me.
Go here ======> MadScentist’s first grow failure.
The whole point of growing on plastic is not having to deal with pH fluctuations, which are so kindly provided by mediums specially like lava rocks. Not going agains’t you @ReikoX, i just don’t wanna see anybody go thru what i did.
I do know that on a no-till enviroment the lava rocks are used and produce great results, but i don’t believe they would work like hydroton or perlite does… On their own. Like the MBBR would.