 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).The water-purifying plants used with constantly submerged, planted, lavafilters (e.g.