Using activated charcoal for orchid medium

[newflasker]

I am testing my home-made orchid medium by adding activated charcoal (AC) for germinating seeds and growing seedling. I don't see any different whether I add AC or not. Even worse, some seedlings grow so slow. Many people said that add 1-2 gram/liter AC will improve root growing but I don't see that way. Anybody has the same problem? What kind of AC are you using and where do you buy it?
Nguyen