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In my experience, my java moss wouldn't grow AT ALL. It did nothing. Didn't really anchor to anything, didn't fluff up, just stayed exactly the same as how I placed it on some wood.
I took it out because it looks rubbish unless it properly grows thick and bushy. As a sort of experiment, and because I wasn't sure what to do with it, I put it in the window, in a cup of cold water straight from the tap and it has grown nice and bushy in the cup. Weird. Not sure what it doesn't like about my tank, but the stories about it growing in any conditions is apparently not true.
I have about 1.25 wpg lighting, hard acidic water. Tried liquid carbon and ferts too.