Volcanic Red Lava Rock as S/H media

[Ray]

As I've said many times - ANY medium can be used for semi-hydroponics, but how well it works for you is going to depend upon your overall cultural parameters. What works well for one may not work well for others.

Unfortunately, flushing alone will not solve lava rock's potential ills. Part of the issue is the accumulation of organic matter and plant wastes, not just minerals. Under the right conditions, that may be a minor issue, but not in most homes.