Lava rock in your pond! Using these washed lava rocks on the pond bed or in the filter encourages bacteria to multiply and significantly increases your pond's ability to self-clean. It also helps the sediment on the bed of the pond to break down more quickly. The lava rocks stimulate a steady flow of oxygen-rich water, which encourages healthy roots and good growth in your pond plants. The lava chunks are extremely porous thanks to their volcanic origins and the extreme temperatures under which they are created. They're also safe for all plants and fish!
Usage instructions for the pond bed:

• 1. Lay down the lava rocks before filling the pond.
• 2. Cover the entire pond bed with a layer 3–5 cm thick.