I use river silt that I dug up just above the water line of the American River, mixed with Soilmaster and topped with more Soilmaster. It has proven to be a very good substrate, judging by my success with the plants. It is certainly a lot cheaper than ADA Aquasoil.
Several months ago Tom Barr did some comparisons of aquatic plant growth using a few different substrates, including Aquasoil, SMS, river silt, and ordinary quartz sand (as I recall). The surprise was how well the SMS performed, and how well the river silt performed, all in comparison to Aquasoil. This was with only one or two plant species, so the results were only suggestive. Again, as I recall, he concluded from this that having a nutrient rich substrate was definitely beneficial, but there were ways other than Aquasoil to get such a substrate. I began using my river silt as a result of this test.