maybe I'm reading it wrong but the Sentence,
Each piece of java moss removed is Nitrate removed.
that would say that java moss is nitrate![]()
So when you remove the java moss its like removing the nitrate. Think of it like the java moss is a sponge. You put the sponge in the water and it absorbs the nitrate, and when you take it out, its less nitrate in your water. But yeah, that sentence is a bit... weird. 
/www.wetwebmedia.com/nutrientcontrol.htm