Zoo Med's Betta Mopani Wood is a natural wood item that is designed to make the water chemistry of your Betta tank closer to what a Betta would experience in the wild. By placing this small piece of Mopani wood in your Betta tank it will naturally leach tannins which will give the water a light to dark brown tint plus lower the pH of your water. (Bettas in the wild live in water naturally tinted by decaying wood and leaves and with lower pH.) Should the water be too dark brown or not to your liking you can remove the Mopani wood and soak in a clean container of water continually changing the water until the shade goes from a dark brown to light brown or clear.
