Agree with others. Leaves do not regenrate, the plant knows not to waste energy and will produce new leaves (from the centre of the crown), but as has been mentioned these too will waste away unless the nutrient issue is rectified. Substrate tabs are the best form of nutrient supplementation for Echinodorus plants, but they will also bnefit from a comprehensive liquid supplement. Some nutrients are taken up by roots, some exclusively by leaves. Flourish Tabs are the best substrate fertilizer I have found.