Yes, you heard it. You can use lava rocks in place of expanded clay. Lava rock can be attained from any chain home improvement store as a cheap and efficient hydroponic stone medium. Why does it work so well? Lava rock is a very porous stone that beneficial bacteria can thrive in and holds water for prolonged periods. Rocks, as many of us already know, are made up of minerals. Overtime those minerals slowly break down and enter the water where they can be assimilated by plants. Volcano rock is known to contain calcium, magnesium, manganese, iron, and basalt which is said to be highly beneficial to plants. Red volcano rock is naturally red from high amounts of iron during the formation, as oppose to black volcano rock which has lower amounts of iron and more basalt. I'm not sure which rock works best as they both yield similar results, I use red since it's easier for me to get. They work great in the garden anywhere you use expanded clay. They can be used as a thin layer on top of air stones to give plant roots anchors and they will not float like expanded clay, they can also be used in ebb and flow/grow gardens. When used on top of air stones they remove a lot of the turbulence the air can create which the roots seem to not enjoy, they also keep root bottoms separated and deter what I call root ponytails which can be the perfect brewing ground for root pathogens to grow. Some people worry about the ruff edges of the rocks harming plant roots, or possibly adding too many micronutrients, but with 4yrs experimenting with these rocks I can see no adverse effects, all positive so I choose to share my successes with you. The amount off added minerals is so minute that the difference is likely negligible, but if there is a benefit to a garden that can be attained so easily and for less cost then I want it! The rocks smooth out overtime of cleaning and reusing if the ruff edges bothers you, but the plants won't mind. These stones are cheap, efficient, and widely available. They add a good home for beneficial bacteria and introduce minerals to your plants, some of which they would not otherwise be exposed to. To clean the rocks you can just soak them in hot water, or if you prefer an unnecessary method of disinfecting the stones you can boil them, bake them, add 35% h2o2 at 15ml a gallon, or even can use bleach as long as you rinse them well. The beneficial bacteria thrives so well on the rocks that running a sterile system becomes unnecessary, it's much cheaper and easier to maintain a balance of bacteria rather than fight nature trying to run a sterile hydroponic system. This works great for people who already amend with beneficial bacteria but that is not necessary to get the benefits. So try them out and let us know what you think. I know if nothing else it can just be a pain to find clay pebbles, but no worries, lava rock is better imo anyway.