ADA Aqua Gravel:
ADA Aqua Gravel is natural river gravels that don’t raise the hardness of the aquarium water. You can spread it directly inside the aquarium tank and use as base sand. (Usable for bottom filter).
This is a natural river gravel which does not raise the hardness of the water. By mixing different sizes based on your aquarium, you can make the layout look more natural. The smallest size, S size can be also spread directly on the aquarium and used as base sand. (Usable for bottom filter) The Aqua Gravel in size S has a grain size of about 2-5 mm.