Robyn,
re: my simple 11 x 16 x [10", 24":]33" pond i build this summer, i dumped in a few green-frog tadpoles, a few juvenile newts, and a few adult newts - and a ton of plants and water bugs.
today, i was cleaning through it and i discovered an adult newt on the bottom of the deep end.
i think that you should not buy them but harvest them as adults and i think they would stay - you said you tried once - but if you bought them, maybe that's the problem - get em natural and past the eft stage and see what happens - it looked well feed and there is virtually no covering on the bottom.
I'm glad you found one. I bought two adult aquatic Eastern newts (the greenish kind not the red efts) around 1999 that I never saw after release. I don't know of anyplace to buy them except for maybe one place where they are about $12 each. Considering that for a few to survive predation, I'd have to saturate the area with them, this would be too costly. I'd also worry about diseases they might spread to my frogs.
Where did you get them? I'd love to try that.