Java fern can be picky and take a long time to establish.. If they get co2 and relatively lot of light it grows substancialy faster. And or change leafshape, starting out rather narrow, but grow wider when leaves get older. They are very slow growers, so i guess Nitrogene isn't realy their main consern, they are much more depending on phosphorus, potassium and micro elements, in long term this average level determine their final grow form and how they show it.
I got a sp. philippine, i ordered it via ebay from a shop located in Malaysia, november 29 - 2015, at that time the only one i could find selling it.
http://www.ebay.com/usr/aquaticmagic?_trksid=p2057872.m2749.l2754
So might as well be you got this one, it definitively is available in the Malaysian aquarium trade..
In the high tech it didn't grow bigger as 4 to 6 cm in a year time, but a lot more mass and a rather smooth and narrow leaf. In the small low tech which i do not fertilise it grows way slower and smaller but a wider and differently structured leaf. They do not realy look like the same fern at first glanse.
That what makes Java fern sp. so difficult to identify in submersed form. Depending on and don't ask what it excactly depends on the most i guess age might finaly tell.
The regular java grows much bigger and wider in the first place and has a vain in it's leaf more buil up like a feather.
And the philippine can shows a vain that build up like a irregular shaped honeycomb and a bit more curled and crackling.
But my own experience tells me, it doesn't necessarely do this, depending on the parameters it lives in, you need to look realy up close. But likely next to it's size the only way to tell from submersed forms is looking at matured leafs. If leaves do not mature and get yellow and slowly faul from the tips but stay attached it might be K shortage, do the get yellow and fall off rather soon before showing dead tisue it likely is P shortage.
It's slow growth rate makes it a very difficult plant to read and a bad indicator.
