I haven't tried it yet, though it's on my to-do-list. But I have herd that lava rock works well. It's also porous, so it gets good aeration to the roots, and it will be a lot less heavy than the gravel. I haven't herd of any pH issues using it, though I would want to test it before putting it in a system. You just want to wash/rinse all the dust out real well first. Also I'm not sure if you read me mention that I usually use coco chips before, it's my favorite
growing medium so far. A 2 cubic foot brick costs about $10-$11 and makes about 15 gallons of growing medium when expanded. I'm also looking into using pine shavings, as well as pine bark because I know they are both commonly used in commercial hydroponic systems.