Difficult to say, there are a couple of factors at play here. As essjay mentioned, the submersed form and the emersed form of the leaves of every species of Echinodorus are different. Aerial or emersed leaves have to be thicker in order to retain moisture in the air, whereas submersed leaves do not have this need in water so those leaves will be thinner. These plants are marsh or bog plants, not true aquatic plants, because they naturally spend half the year emersed and half submersed, or at least they can; in many of their habitats they may be exclusively emersed. The other factor though is the conditions. I have had adventitious plants (the "daughter" plants) of the same parent sword plant grow very differently depending upon the tank each plant was growing in. These are heavy feeders, with extensive root systems. Several years ago I tore down my largest tank, a 5-foot 115g, and the roots from the sword plants were thick throughout the 4-inch sand substrate, literally holding the sand together. Give the [lant good nutrition, and here the substrate tabs are best. Flourish Tabs are one of the best, the API tabs are not as good and can be very messy. I'm sure there are other tabs as well. But a Flourish Tab inserted close to the crown and poked into the substrate, replaced every 3 months, will make a real difference. If the existing ;leaves are emersed, they will die off over the next few months, as new growth appears, which will obviously be the submersed form.
