The best growing plants I have ever seen tended to be rooted in smooth
and heavy clay soil. The plants really seem to root well into it, and
the soil doesn't seem to vanish as fast as other forms of soil. I
made the mistake of buying some of that Schultz aquatic plant soil
thinking it was chips of clay and it was an absolute disaster. The
"soil" is so light that nothing would root into it and the plants all
looked pathetic and weak.

I've never seen clay for sale anywhere. What about clay that is used
for making crafts? Is that safe for aquatic purposes?