Unfortunately the picture quality makes it difficult to tell to what exactly is going on.. nor do we know anything about the husbandry you are providing them. However, the symptoms you mentioned makes this issue pretty serious. Also, a foam filter won't do anything to get rid of nitrates...only water changes and plants can do that. foam filters just provide a media for bacteria that converts ammonia to nitrite and nitrite to nitrate to colonize. So you just didn't stay on top of your water quality and parameters. This species, like many stream species, requires very clean water...deviations from this will depress their immune system and are likely to become infected by opportunistic fungi or bacteria..causing septicemia.
Personally I would have just changed the water and and given the inactive one baths in amphibian ringer because most systemic infections lead to electrolyte derangements..I also would have gotten the newt swabbed and tested to narrow down the pathology.
Unfortunately two weeks just isn't enough to know if your newts are "doing great". If this one passes you should get a necropsy done so you can narrow down exactly what your newt got infected with. This may in turn help you save your other newts.
Also, moving your newts that were mainly aquatic to a terrestrial set up abruptly comes with its own set of issues, especially with this species, so be mindful.
This genus can be tough and it sounds like you need to do more research regarding their care, which can be complex and still not very understood. There are papers out there that can guide you in the right direction. Also, as stated before, stay on top of their water quality and parameters.
Hope this helps.