Lots of clay options, if you're thinking of potting clay / red clay for nutrients I'm sorry to tell you, but that's a myth and won't really provide much of anything.
Use DIY Osmocote+ tabs instead.
I'm with Quagulator on this. I did a lot of work with clay-based substrates in the past, and while they're useful for going plants for habitat restoration, they're horrible for aquarium use. If you're interested in substrate as a nutrient source, tabs or Osmocote+ are the way to go.
wouldnt the pottery clay at least have some cec?