Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Kitty litter has high CEC because it is often montmorillonite clay.
Many kitty litters have a lot of dust, or are so soft they fall apart in the tank. And we are certainly not using the perfumed ones.
However, I find Safe-T-Sorb to be a lot nicer looking, and quite reasonably priced. It is also a montmorillonite clay, but is baked a bit harder than some of the kitty litters, and is a blend of soft tans and greys.
I would not call them inert.
These materials are all active in that they will hold fertilizers in a way that plants can take them, a lot like an ion exchange water softener. (CEC = Cationic Exchange Capacity)
They will remove the KH from the water, so some supplement may be needed.
They are too light weight to support a sand cap. The sand will simply sift through the particles and end up on the floor of the tank. It may stay blended with the material for a while, but every time it gets disturbed the sand settles out faster, so gets closer to the bottom.