mixing charcoal into the gravel?

No, it wont change your water chemistry. However, I can not think of one good reason for doing this and can think of many reasons not to.
 
Activared carbon is short-lived chemical filtration with an effective life measured in hours to days. How are you planning on removing it?

Activated carbon is very friable, every time you vacuum you will generate fines. Eventually it will all be fine black dust. Very nice appearance and very good for the fish - not!

Not a very good idea.
 
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