Join Date: Feb 2008
Location: Blue Springs, MO
From what I understand the difference between "sand" and "gravel" is just size and porous content. So as long as you stay in the 1mm to 3mm range (which is nearly all the substrates we use) your fine on CEC.
What Hoppy said earlier is spot on for what I've seen myself, anything smaller that PFS (like play sand's .05) becomes low in CEC and if you get above 3mm the same thing happens.
For my personal use PFS, Flourite, Eco, SMS/Turface, T-Grad Colorquartz, and SAS grow all the same with a good fert regiment + root tabs.