I would steer you away from using charcoal (like that used in grilling) and towards "activated" charcoal/carbon. If the cost is cost prohibitive with activated charcoal, you may be happier if you were to save your money and avoid the use of carbon completely (a largely optional component of reef husbandry).
Contrary to what some youtube tutorials might show, I would not suggest you try and "activate" your charcoal at home