Appears to be Echinodorus schlueteri transitioning to immersed form from emerged.
If that is a new plant or you have been growing it emerged then I am reasonably sure it is Echinodorus schlueteri.
Off the top, it appears to be Potassium deficiency though these things are hard to say for sure. It looks as though it could use a little more iron/manganese, maybe a lot more depending on how long you have had the plant. :gw