It happens to me with Echinodorus ozellot green. Old leaves usually show yellow patches when they are two or three months old and if not trimmed they become ugly (brown, then darker and finally a hole). I though it was a kind of deficiency but the plant is producing new leaves very fast. I guess it is a mix of lifespan and difficult situation to have good light and nutrients (these leaves are below the new ones). As the young ones are really much more attractive (contrast between green and black marks) I just prune the old leaves once they begin to show signs of senescence. The good part is that this plant grows too much for my tanks so I keep it on a kind of permanent juvenile stage. Not sure if I will exhaust it...
Jordi