I haven't ever heard of Cynops/Hypselotriton (firebellies) causing the water to become toxic, but I have heard of frogs and toads being killed when sharing water with stressed Triturus (crested newt) species so it could well be possible for Cynops to have the same effect.
In my experience captive Cynops newts will only resort to their Tetrodotoxin defence when they feel extremely threatened. With the firebelly species I keep it's recently morphed Juveniles that seem more likely to secrete poison, but by the time they've matured and become relatively tame it almost never happens.
Hope this helps.
