Hi CG. I had the same problem with adult T. marmoratus before. Regardless of how much they ate, they never fattened out as hoped...and they eventually died.
And I've had the same problem for a year or so with one of my P. waltl, an acquired individual I've isolated from around 20 others I've bred but kept. It eats and eats but remains skinny -- skinny but alive. I just try to compensate for whatever the worm/worms are eating by feeding the newt frequently and as much as it will eat in one session.
In the case of my T.marmoratus, before they died I'd contemplated feeding them discus burger containing a dewormer, figuring that if it was safe for such a delicate ornamental fish it might be safe for newts. But it's only effective for a specific type of worm (intestinal I think).
