There are some (Amano and others) which use active carbon in the substrate for mainly 2 reasons :
1. absorbing any (hopefully all) substances that could enter the water column - tannins, pigments, organic molecules all of which can come from peat and others.
2. serving as extra space for bacterial growth, because of its big surface area.

I found a ref. in Biohome web site regarding the use of carbon in the substrate which states :



I'd appreciate any answer regarding 2 main issues :
1. The use of AC in the substrate.
2. Does AC's pore size is really too small for bacteria to settle in?

Thanks

It's pretty much true, everything you have said to some degree.

The real question is how long it last.

Well.........most Ac is spent, losing it's absorption capacity after about a month.

Then it's just biomedia, generally gets pretty clogged up, like most fine pore biomedia. Doesn't mean it's not useful.

It acts as a "sticky" for nutrients, but unless the roots or reductive bacterial processes remove it, the nutrients, they should stay there.

Once removed, there is now a new site for a nutrient to attach to.
Leonardite is somewhat similar, it's peat=coal mix which is very much like AC in some absorption ability.

Regards,
Tom Barr

Thanks Tom, app. it.