Some plants sold will nearly always die off but others can be saved. To increase the chances of survival find out when the new plants arrive in the shop and buy them before they have been in water too long. The structure of the leaves is different between emersed and submersed. Put the stems in to water and give them strong light until they grow roots. Most people want plants that are red so buy Echinodorus red special or similar plants that are naturally red and are aquatic. They will grow red with the right fertilizers and good light.