I buy my worms (Canadian nightcrawlers) -- they are dye / chemical free. I know the 10-gallon is small, but with the 2 being only 6.5 - 7" max (just measured the one, the other one was about the same size) I felt ok with using it for temporary quarters until I got them a larger habitat. I've only had them a month or two. I have been feeding generally every other day depending on how they chubby they looked, usually 1 2-inch piece of worm but then the little beggars would often get another piece of worm out of me.
I'm wondering if when he/she ate it's piece of worm the other night (Sunday) it ingested too much substrate, or the substrate caused some sort of blockage. The worms get some coco fiber on them while the salamander is eating it.
Any ideas to maybe help me figure out why the little one died would be appreciated.