Rockwool trays are designed to be compatible with Grodan Rockwool products and work especially well for drip and ebb and flow hydroponic systems. These trays work differently than others, mostly thanks to their molded layout. While you can use them with any media, rockwool cubes …will work best. Rockwool is an inert pH stable planting media, made from furnace blasted and spun stone. Think of it like cotton candy, but made with rocks. It provides an excellent balance of oxygen and water saturation in the root zone, and integrates well into any drip hydroponic feeding setup.