Well first of all carbon is carbon chemically. The biggest difference between a charcoal brickette and an Activated charcoal lump is physical, the brickette is usually compressed, the brickette may also have some binders, and some things to help it light like wax, oil or even sodium nitrate, both are probably not good to put into your pond. Activated Charcoal is usually the product of pyrolysis (heating without oxygen) of wood or coconut fiber and is a light porous material, and has a surface area on the order of acres per teaspoon. AC is very good for adsorbing chemicals from a fluid like air or water. The brickquette being compressed will not adsorb anywhere near as much as AC would.
If you're going to us a grilling charcoal, get one of the more expensive ones that still looks like wood with visible grain and pores, and doesn't have any adulterants added to increase light ability. Personally I'd either us AC for lab or industrial use or make my own so I'd know what's in it.
