What Do You Put Under Sand?

[xFlx]

What is a good substrate to put under a layer of sand? I've heard loam and soil..but is this safe for fish? what's best for both fish and plants?

 

[aaronnorth]

I like JBL Aqubasis+
SOme use soil, but make sure it is soked first, and then baked in the sun to remove the ammonium, as it will leach & cause algae.