What do people use to separate substrate from...say, decorative sand?

[neueklare_ein]

I am pretty sure that thousands of people have asked the same question. But what exactly are those plastic...thing (usually black)? Can I buy that online? Can someone show me some photos of how those things actually look like? I am very familiar with paper or paper-made stuff but I doubt if card board can work under water over a period of time at all.

Where should I buy that kind of "separator" ?

[acitydweller]

Are you referring to the black plastic edging for gardens ? These are available t home depot, lowes and wlmart.