I need hardly respond to these questions, there are so many disciples.
Everyone is correct as far as it goes, but in this case I would suspect the issue is not nutrient-related. Assuming you bought relatively newly-arrived plants from the store, chances are they were supplied by one of the nurseries (Tropica, Florida-something, and some others) and these grow swords emersed because it is quicker and less expensive. If you have a look at the plant profile for these species [click on the shaded name in posts, example Echinodorus bleherae
, or use the second tab from the left in the blue bar] you will find that the Echinodorus genus are amphibious bog plants in nature, spending half the year emersed (the dry season, when they flower) and half submersed (the flooded months). It also notes that the leaf forms are different between these two forms; this is because leaves that are emersed have to deal with air rather than water during exchange of gasses and water.
When you acquire the plant and submerse it, the leaves on it will be the emersed form and slowly yellow and die, and they should be removed once yellowing begins. They will never recover. New growth which arises from the centre of the crown on all Echinodorus will be the submersed leaf form, and provided you have new green leaves emerging, the plant is fine. Expect all existing leaves (when you bought it) to yellow and die, unless some may already be the submersed form.
Sometimes leaves will do the same (yellow and die) if the change in water parameters is significant between what they were in and what you put them in (pH, hardness, less likely temperature). I am not a botanist so I don't know how significant this change has to be, but it is always a possibility with almost any aquatic plant.
I agree with whomever above said nutrients from Eco-complete and Flourish would be sufficient. However, having never used Eco-complete, I have no personal knowledge of the extent of nutrient release in it, and as someone mentioned, Echinodorus are very heavy feeders. But as I have kept beautiful swords of several species with no fertilization other than Flourish Comprehensive twice a week, I would think that using Flourish once a week plus the enriched substrate might be sufficient. I would give it a couple weeks; if new
growth starts showing nutrient deficiencies (yellowing usually) then increase Flourish to twice weekly, the second about 3 days after the first, such as Monday and Thursday. And always wait one day (24 hours) after a water change with a conditioner that detoxifies heavy metals, as most do; some heavy metals are micro-nutrients (iron, manganese, copper, nickel, zinc) and there is no point in dosing with Flourish only to have the conditioner in the water detoxify the minerals before the plants assimilate them. And conditioners generally remain active for 24 hours, possibly slightly more according to what the chemist at Seachem told me when I asked about this.
And I agree that dosing more than required is dangerous. As just noted above, some of these minerals are heavy metals, and all heavy metals are highly toxic to fish and plant life at sufficient levels (beyond "trace"). Many including myself have killed or severely decimated plants with overdoses of this or that mineral, which is one main reason I do not recommend dosing individual nutrients like some authorities suggest. Not to mention the loss of fish that could occur.