Sorry to hear about your newts :(
I would try using water from the pond, then put lots of plants in, if possible try to use rocks, substrate and plants found in the pond. I find that something magical always happens when you use substrate from nature, tanks seem to cycle right away ;)
Test the water params periodically, and if they're not off I wouldn't change it. I do this for on the verge of death bettas and they take much better to it than water changes.
Eventually the TDS will go up, but you shouldn't have to worry about it for weeks... ( I've left my shrimp tank on its own for a couple of months without water changes and they did surprisingly well...)
For waters changes (because you'll have to acclimate them to tap water sooner or later right?
If you have planted tanks with across the board 0 readings, I would use that water for water changes... it may sound weird and counter productive, but the water in your planted tanks have their own little eco system in it as opposed to the sterile tap water we usually use for water changes.
I would recommend changing out very little at a time (maybe 5%?), and your newts should acclimate over time. Wild creature are often very delicate and don't do too well in captivity unless we imitate their natural habitats, from KH, PH, GH... etc.
Sorry for blabbering on!
looking forward to any future newt keeping endeavors :D