Best substrate?

[Kevin1]

Your very welcome.
Please post plenty of pics when it gets finished.

[SludgeMunkey]

I like to throw a handful of play sand into the mix also, helps to keep it from "chunking' up over time.

[Jace]

Play sand? Okay, I'll do that as well. Never hurts to try it. And I promise to post lots of pics!

[Douglas]

Bark is a very bad choice...good to hear you're removing it.

I like about 2/3 cocofiber and 1/3 organic topsoil. The soil helps to hold onto some of the moisture better then just just cocofiber. As as kevin alluded to, a mix provides more stability for live plants to become rooted.