Eel grass and Java moss are a good start, but any fast growing plants will do just fine, the bunches of cheap pondweed looking stuff that most petstores sell for a few bucks are perfect. The faster growing they are, the better they are at sucking up waste from the newts, so the better the water quality will be.
I know I already mentioned it, but don't underestimate how much Chinese firebellies hate water movement, deflecting it off the glass isn't normally enough to diffuse the outlet of an internal filter. I would urge you to either fully diffuse it through a piece of filter sponge or similar, or remove it altogether, at least for the time being. Newts aren't like fish, they can vote with their feet, if there's something not quite right with their set up they will sit on their island looking miserable, or spend all their time trying to escape.