I wanted to say N was to high but, It just sin't. Notice there is no claw on the ends. The color seems like it's a bit dark but, it's not to dark nor is the leaves shiny or waxy looking.
The newest growth doesn't seem to carry the "curl", and the "curl" as it is, is not a sign of N tox. It also points to not over watering - yet those leaves sure have that look. Be sure it dries out enough before watering is the simple answer - Flowki gave that already.
Heat is a choice also......What are the room temps or ambient temps where the plant is? Day and night please. How about the RH or Relative Humidity of the area?
That's not N tox, yet.