A carpeting plant might be a little hard for a a low light no co2 setup. You could tie the java moss to slate and let it come through a netting to simulate a carpet plant. If you do want a carpet, maybe use flame moss instead of java since it grows straight up. I don't know the requirements for flame moss though so you might want to look up what it all needs. You could also try riccia for a carpet. I have it growing as a type of foreground plant in a .9 gallon tank with a 13w cfl.Also The pic as my avatar and profile pic are a 2.5g and it just shows you that even though it is a small space, you can do a lot with it.