Just adding to this thread,
I'm shortly going the whole pressurised co2 way.
I put down a base of Aqua Basis 6 years ago and am wanting to do a good replanting of my 4foot tank. Presumably the substrate is well and truly past providing any benefits, but is it still necessary to put down a new substrate layer if using co2? I thought if I did I'd use the Hauraki Gold Peat Moss as mentioned earlier.