These are constructed out of 2 layers of lava pebbles and a top layer of nutrient-free soil (only at the plants roots).Example picture of constant-submerged lavastone filter On top, water-purifying plants (as Iris pseudacorus and Sparganium erectum) are placed. Usually, around 1/4 of the dimension of lavastone is required to purify the water and just like slow sand filters, a series of herringbone drains are placed (with lava filters these are placed at the bottom layer).Best Water Filter To Remove Calcium and Lime
