Java fern doesn't grow that quickly. In order for a plant to take care of waste in your water it needs to grow quickly.
A great list of plants that WILL help keep your water clean are:
1) ANY floating plant (other than carnivorous floating plants)
examples: frogbit, red root floater, duckweed(comes in greater and lesser), water spangles, and water lettuce.
3) Water Sprite
4) Mexican Oak leaf
These plants grow really quickly as in you prune them once a week. because they grow quickly they suck up a lot of nutrients (to them, toxins to our fish)/ a TON of bad stuff out of the water. You still need to change the water often but not as often as you would without them. =]
A great place to buy any of these plants is this site right here
You can also find these in your local fish shop if you have one. CHain stores usually wont carry them but then again the only time I get to visit a petstore is when I'm back home in a rural area.
I have no idea what Urban chain pet stores carry. :P
ALSO these plants are typically classified as floaters and they get all of their nutrients from the water around them so they don't really need any enriched substrate to survive though you should try to fertilize with liquid fertilizer (traces) every so often. So you can just throw them in there tie them to a rock and let them sit in your tank, or poke them into the substrate to create the illusion of a rooted plant. =]
If you get a legit floating plant though (anything from #1) be advised that they multiply RAPIDLY and will most likely shade your entire tank in a few days to a week or so if you fertilize. =]
ALSO if you get any
of these plants you'll need a daylight (not soft white or white, a daylight
) bulb that's a low wattage (you want about 2 per gallon). =] The amount of time you leave the light on depends on how many watts it is and if your tank gets any sunlight. If it's around 2 wpg w/ no sunlight then you can leave it on for 8 hours. If it's more than 2wpg or it's 2wpg and gets sunlight as well, leave it on less. You don't want algae. =]
I copy-pasta'd from another post of mine in a different thread because I'm LAZY. :P
Oh! to balance all of the bright colored fast growers I recommend that you add a few dark slow-growers. =] That way your tank has a nice variety instead of all light green! XD