Hydrotons, or Clay Pebbles, or LECA (Light Expanded Clay Pebbles) are an excellent growing medium in Hydroponics, Aquaponics, and Soil-less Gardening. CityGreen's Fly Ash pebbles are an economic substitute for Clay Pebbles. Instead of clay, these are formed using Fly Ash. They are much lighter than clay pebbles and suitable for growing microgreens and in systems where weight is a constraint (like DWC). However, unlike clay pebbles, Fly Ash pebbles do leave residue behind and thus if you are planning to use them in a recirculating system like NFT or dutch buckets, do use a filter to avoid clogging the pump. They are an excellent medium to be mixed with soil or cocopeat for improving the water holding capacity and aeration in the growing media.