Natural substrate rich in carbonates. Non-chemically treated soil. Onyx Sand is a natural dark gray sand designed for the planted aquarium. It is very rich in magnesium, provides iron and other minerals, but most importantly supports optimal KH levels. Due to its high carbonate content, Onyx Sand offers all kinds of plants the advantage of being able to use bicarbonates for their growth. Onyx Sand has a slight buffering capacity, which can slightly increase the pH. It is a very porous soil, which does not clog and has a long life span.