Growing substrate for oxygenating plants 8 kgA natural starting aid for the pond! The Ubbink OXY (growth) substrate consists of porous stones with various shapes ranging in diameter from 2 to 8 mm which are spread on the bottom of the pond or in a plant basket. They promote the nitrification process, supplying the nutrients and carbon required for plant growth so the plants can produce oxygen. This ensures that you have a lovely, clean pond!