Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Well, you could use all Moon Sand, and put fertilizer tablets deep in the sand.
Like other sand, it does not have any cationic exchange capacity.
Substrates with high CEC and are safe for Cories include Turface, Safe-T-Sorb, the ADA products and others.
Mineralized top soil is likely to be Cory-safe, but can be so fine that it clouds the water if they are really serious diggers. (My Cories are not, but I have some Loaches...)