Hydrofarm GROW!T GMC25L Clay Pebbles 25 Liter Bag, 4mm-16mm Made from 100% natural clay, extremely stable in both pH and EC Pre-washed to aid in stability Drains freely and does not hold excess water, provides good oxygen levels around the root; suitable for flood and drain systems, multi-pot and drip feed systems Strong structural integrity means that they are less likely to break and compact, which helps to preventdripper systems from getting blocked Not sold in OR