WRT to the "sand cuts the gnats and/or their larvae", the core idea is sound, but the implementation is flawed. It's diatomaceous earth that is an effective insecticide by virtue of scratching through an insect's waxy epicuticle, subjecting them to gradual desiccation and eventual death. I'm not sure if this works on larval exoskeletons. -- Rodger Whitlock Victoria, British Columbia, Canada