Over watering is the most comment mistake people make in caring for their house plants. Horticultural charcoal helps combat overwatering a prevent root rot, both by absorbing water and killing fungi and bacteria.
It doesn't take a huge amount to get the benefits that charcoal provide. About 1/8th a cup per pound of potting soil is all you need.
This listing is for 1/2 half a pound of horticultural charcoal.