[brenmuk]

The best tank I've seen java fern grow in was in a neglected tank in my old work - it grew in the substrate and on ornaments -everywhere in fact it was the only plant in the tank. The tank only recieved a few hours of artificial light from an old tube while the rest of the time the light came from the lights from the office (not ideal texbook conditions but the java fern loved it).

Lets face it if the conditions are right for other plants to thrive like Blyxa japonica then you must be providing the basics for good plant growth. If some are thriving but not java fern then it may be at a disadvantage in terms of nutrient competition/allelopathy - so it might be the combination of plants that you have. Java fern if attached to rocks and wood does not feed from the substrate so that is one potential disadvantage it has compared to other rooted plants.

If I were you and I might be tempted to remove the Blyxa japonica from the NPT (as an experiment) and see if java fern takes off. What you might find is that the next most dominant plant starts to thrive and take over but the java fern might pick up a bit. You could keep removing the next most dominant plant till you end up with the combination where java fern thrives.

In my NPT I have gone about things in a slightly different way I have chucked loads of plants into my tank and kept the plants that seem to grow well together.