Well, there's Aquasoil and then there's Power Sand.
Amano puts Power Sand down as a first layer and then covers it with Aquasoil. Power Sand acts like substrate fertilizer and Aquasoil acts like substrate. I never use Power Sand, just Aquasoil, so I'm not adding all that nutrient stuff. Power Sand may give an initial boost to growth during the fill-in stage and it can cover for people that don't dose routinely but it eventually runs out of nutrients and then it just becomes a base layer. I have no idea how long that takes to happen but when it does you'll either have to add substrate ferts or start water column dosing. Amano offers both. I use Aquasoil which is cheaper than Power Sand and just water column dose from the get-go, in the traditional style. You can always add mulm before the Aquasoil to get the bacteria-boosting effects.
As far as EI water changes removing the nutrients from Power Sand, Power Sand is meant to supply nutrients to the roots, not the water column, which is why it's buried under Aquasoil, so Ei water changes shouldn't be affecting that much. And yes, they say the larger grain size of Power Sand improves water circulation at the root level. Aquasoil works just fine in my low light, non Co2 tank. I run it like I run all my others except perhaps lower fert levels. Mosses and crypts.
When using Aquasoil only, you're getting the benefits of the substrate's properties rather than the fertilizing effects of Power Sand. To me, Aquasoil's benefits are appearance, grain size (runners grow easily and in more even paths, grain size is more uniform and scales down to nano tanks well, it's easier to plant in), aquasoil doesn't easily crumble under normal use, yet it's soft enough not to scratch your glass, and it just plain grows stuff better.
Tom or Jeff Senske or someone else can give you a better chemical description of how Aquasoil works, ion exchange or some other such scientific stuff.
