If your nitrate is indeed 0, then that might be the problem. Plants need nutrients, and if they are amiss, then they will still grow, but basically loose their existing substance as fast as they build it.
Your plan sounds good, but also look into supplying (very, very little) macro nutrients NPK if your test kit is correct and N is zero.
Is that plant anchored to a piece of wood or rock? The base of the plant is not visible in the picture. But make sure you are not burying the roots & rhizome....this is a common mistake that will ultimately kill java ferns of all varieties.