Many people don't use carbon at all, so how bad can the stuff (excluding meds) be that would be leached back into the water, when in many tanks its not even "removed" in the first place? Carbon works via the van der waals force, which means there is no real chemical reaction that takes place - what goes in is what goes out.
As for going out, different things have different levels of attractiveness to the carbon, or affinity. Things with a high affinity for the carbon will not leach back into the water because of the strength of the bond and the amount of energy required to break it. However, once the attachment sites are all used up, things with a lower affinity for the carbon can be bumped out by something with a higher affinity.
Like the others, I would not put it in the substrate either. With the ammo chips - your tank ought to have sufficient surface area to support a bacteria colony to handle the bioload without the aide of ammonia adsorbing products. It's one thing to have them on hand for an emergency, but quite another to use them long term.
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