It sounds like you definitely have a cynops p. which is GREAT! They're more hardy and I have three of them, therefore I'm bias. Plants are great for an aquarium, they turn nitrates and nitrites into a nontoxic form. You need your Nitrate, Nitrite, and Ammonia tests, which you can get at petsmart or any other store accompanied with fish. Probably not walmart though. I'm not sure if plants do much for the ammonia, but all of those can kill your newts. Though I'm sure the setup has had time to create a good biofilter. Just remember, the strips of paper are notoriously inaccurate, get the tests with the small tubes and chemicals.
What kind of substrate do you have now? Simple gravel is best if you're gonna be planting, you can find all kinds of aquatic plants online, the best are java fern, java moss, and anubias which all thrive in low light. Those are the three favorite plants of mine. The newts like resting on the anubias leaves. You can also invest in some aquatic grass and maybe densely plant an area of your tank. I bet you they'd love it. Japanese fire bellies love densely planted environments.
BUT this means you WILL have to hand feed them, because you don't want to siphon a ton and mess with all the roots and whatnot. I've been having trouble hand feeding mine, but in the end; four weeks later, all three are eating from the tweezers. Funny thing about my newts, Ra and Pete like blood worms and earthworms. Shadow ONLY takes earthworms... SO what I suggest, thawing a blood worm cube in some of there water in a small dish, pick out the good pieces, nice and fat, and dangle the worm in front of their noses. Newts have GREAT eye sight and might become afraid of your huge form, it's possible they won't take the first few times, BUT the instinct to eat always prevails. IF the blood worms don't work I would suggest to you a great tip, try earthworms. If you cut off the tip it will keep moving you can see if the movement makes them bite. Maybe you can even place it in front of them and walk out of the room. After they take the first bite their bound to take the next, that's how I got my toughest egg to crack.
Update us soon. Lets see some pictures, you would be surprised what kind of suggestions people might come up with or some observation that might be made.
