Gold Label Hydro expanded clay pebbles have a very solid outer ceramic layer, which limits the uptake of water. They are ideally suited for intense irrigation hydroponic styles of growing. Be aware that though the pebbles are very dense, they can float in deep ebb-flow systems. We recommend top watering systems for Hydro. For ebb/flood systems we recommend using our Gold Label Hydrocorn clay pebbles. As the pebbles are round and have a very even shape they leave a lot of space for air in the root system. The closed even pebble makes it hard for the roots to attach, enabling easy cleaning and re-use of your substrate.