This soil amendment is a must-have- whether for your terrarium, or to add into your potting mix! Horticultural Charcoal has toxin-removing properties, as well as odor-removing abilities, and when used in soils, can help prevent build up of bacteria that cause root rot, smelly decomp and more. There's really no downside to using Charcoal in your mix!
Some use as little as 5% to 10% when mixing their soil. Others use a 1-2 inch layer to act as a an odor absorbing layer that also filters out impurities.