Very interesting, so would it be fair to say any medium with a high CEC will tend to draw the carbonates/KH out of the water and exchange it for something else.
Yes, it’s basically CEC binding sites are negative charged and they attract positive ions, even other negative ions once they bind with a organic acid such as humic/fulvic acids or chelating substances you add with ferts they can bind to CEC sites. Root hairs and multitude of fungus/microorganisms there then use gas passed to roots or pushed by currents into substrate to perform the actual nutrient stripping from the CEC sites. Oxygen is released into substrate as a byproduct of that process. Then CEC site then immediately becomes available to be repopulated by another ion or compound.
Really in substrate choices these are broken into 3 types of choices.
Inert substrate, 0 CEC value until say 6mo to a year when a bit broken down organic matter builds up in substrate from plant decay/poop etc. Still a low CEC value when mature but enough to get by on. Really takes very little water column dosing to saturate that small CEC potential. Mild, mild buffering effect, barely even noticeable.
The clay based stuff like Fluorite or volcanic types like eco complete. Very mild CEC value right out of bag. Again buffering effect almost unnoticeable.
Then soils, which have a extremely high CEC and buffering capacity. You just dose what you know you should be dosing per week and don’t worry about it being totally gone and unreadable on test kits next day. Those nutrients are there to be taken up roots of plants or released by a say a influx of distilled water on top off.
Either high or lower CEC substrate are your choices. If you want a true med CEC bed in your tank you pretty much have build it yourself by mixing something like coco peat 1 part to 4 part 1-3mm sand on bottom layer, then cover it with 1-1.5” of sand. You’ve just built a sandy loam substrate layer, true med CEC value but not the huge buffering sponge of soil balls. Just like a Walstad type tank organic decomposition in tank along with some light dosing will replenish that CEC layer with nutrients.