Charcoal IS the substrate in some springtail cultures. It is the living space. it makes it easy to grab a chunk out and tap it into the tank, knocking off the springtails. It makes it easy to flood the entire springtail culture in water. Then the springtails float while the charcoal does not and it allows you to easily pour the water and springtails, together, into the viv. In addition, charcoal is a purifier and it keep s the culture fresh.