The truth is that almost any kind of medium grained substrate would work great. There is a growing collective opinion that these specialized "plant" substrates really aren't as effective as the manufacturers want us to believe.
With amazon swords, Eco Complete would be great but the truth is I have seen some amazingly huge and robust specimens growing in plain old gravel. And they are heavy root feeders, so if you give them Flourish root tabs they will be very happy. Mine grow in Onyx Sand with the tabs and they doing just great.