Plants rooted in the substrate will grow fine in almost any substrate. The grain size can make a difference though; too large (above pea gravel) is not good, and even pea gravel can be problematic for some plants (I had a tank with this gravel). A fine gravel or sand works fine. I have also used Flourite (which is basically identical to the Eco Complete mentioned previously) and I have used a laterite layer under the gravel. In both cases, to my observation the plants did not fare any better than the same plant species in other tanks with plain fine gravel or sand and identical light and fertilization. Personally I wouldn't waste the money, though I will say that as you are intending diffused CO2 this might make a difference, but I tend to doubt it.
It also depends upon the intended fish. Some of these enriched substrates can be rough, as are both Eco Complete and Flourite. Corys for example should not be housed over these; I got this from a catfish expert some years ago, but sadly didn't listen and damaged my corys over the Flourite, which I chose because it felt less abrasive in my hand than EC. I can't speak for some of the other products, like ADA and such, as I have never used them. Substrate fish tend to fare better with sand, thinking here of corys, loaches, many loricariids, and some cichlids that are substrate feeders. I now use play sand in all my tanks, except for fine gravel in one.
Whatever the "enriched" substrate, I would seriously question the actual benefit. All aquarium plants have to take up nutrients from the water, via roots and leaves. Those in the substrate can benefit if nutrients are present in the substrate because these then dissolve into the water and are taken up by the plant roots. This is primarily why any substrate material works. You can always add nutrients to the water column via liquid fertilizers and substrate tabs without the issue of the "enriched" substrate giving out. Diana Walstad admits that after a year, there is no benefit of soil over sand/gravel because by this time these will have developed the necessary organic base to sustain the plants. Some of the enriched substrates do affect water chemisty, either short-term or more long-term. I noticed that Flourite did keep my pH a tad higher for several months. I tend to prefer inert substrates to avoid one more factor of interaction.
Byron.
