I probably wouldn't be using Java Moss to carpet a tank unless you are a true newbie. You can get much nicer looking mosses that carpet better such as Christmas Moss. I also have a ton of Spikey Moss that I use which carpets really well and looks way better than Java. In my experience, Java Moss takes way too much maintenance.
Java Moss is very forgiving. I once cleaned out a tank of the stuff threw it in a freezer bag and left the sealed bag out in the sun for several weeks in the summer. The moss in the middle of the bag had died but the outer layer of moss had doubled in size.