As far as the charcoal goes.... how are you intending to use it? And what type charcoal? I used to use granular activated charcoal in an aquarium filter. It was a re-fillable canister, seems like I replaced it monthly, but that was a very long time ago so I'd have to look up the manufacturer specs.

I don't think activated charcoal filters out very many dissolved minerals. It is more for dissolved gasses.... I think.

If you are using it as a substrate, I wouldn't think the mineral loss would even be an issue as your water is just touching it, not flowing through it. If it's just color you are thinking about, they have a black sand in the aquarium dept.