Hi Topfrog, I live in Liverpool, UK and my water is soft (no limescale). If you look at some of the videos they do, do layers large gravel/Kanuma on the bottom drainage, then (I think) a mix of Akadama/pumice/lava (middle layer) and then a fine top dressing of Akadama, that's why I ask about charcoal in the drainage layer and its benefits to the trees.