Go to caudataculture.org, and there is a great article on how to identify fire-bellied newts.

I would go to a different moss,such as feather,or sheet moss would be good as sphagnum moss is often acidic. I would also add willow or java moss to the water area.
It would be a good idea to add some more hiding spots,such as pieces of bark,rocks,or small pieces of wood. Dead leaves from a safe-source on the land area is another good option.

Great food items would be live blackworms,redworms,waxworms,bloodworms,chopped-up earthworms,slugs,pillbugs,fruit flies,and springtails.
These can be offered in a food-dish or held in front of the newt with tweezers.

You can add a calcium/vitamin supplements. Tetra,Zoo-Med,or Fluker`s are good choices.

I hope you the best of luck!