Originally Posted by SingAlongWithTsing
wouldnt the pottery clay at least have some cec?
If you use legitimate clay rather than a synthetic it'll have some. I highly recommend *against* using clay in aquariums though. It's actually pretty awful for growing plants. If you've got access to it, check out my talk at the 2015 AGA convention where I go over the substrate portion of my old research. Most of the treatment mixtures were clay based and ended up being completely unsuitable for aquarium use.