Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Build a barrier with some small rocks and expanding foam filler for ponds (it is black, and is fish safe.)
Make an enclosure near the front of the tank, any shape you want.
Just be sure it is sealed off from the EC.
Then fill the enclosure with pool filter sand, and the rest of the tank is EC.
When substrates are different weights the heavier one tends to sift itself through the rocks and ends up on the bottom of the tank, while the lighter material can drift over the rock barrier and end up on top of the other material.
I have made rock barriers without any foam or caulking, and the materials end up mixing, with the sand (it was heavier) seeping through and under the barrier.
When I used the foam the sand stayed in place. The other substrate still drifts around in the tank a bit and lands on top of the sand, but it is fairly easy to clean it off. (The other substrate in my tanks is Soil Master Select, a lot lighter than EC) I would think EC would stay in place pretty well.