If the newt died in 20 hours, it was not the fault of the feeder fish (and not your fault either). A healthy newt can live for weeks without eating, and can survive all kinds of poor treatment for a long time. You got a newt that was so badly mistreated that it didn't have much life left by the time it got to you. Unless there was something toxic in your tank, that's the best explanation for why it died so quickly.