LECA - Lightweight Expanded Clay Aggregate
Leca clay pebbles are often used as a growing medium in hydroponic systems because Leca is very porous. Leca is baked clay balls which can hold water and provide it to the roots of the plant when necessary. Leca is an inert grow medium, it contains no harmful substances or gases and is absolutely neutral. You'll have to add nutrients yourself, a hydroponic nutrient solution is recommended. Besides the water holding capacity, using Leca also reduces the risk of getting pests due to less chance of rotting roots.
Leca clay balls are used in aquatic filtration systems as they provide a larger surface area for beneficial bacteria to colonize and grow.