Anything specifically formulated for amphibians should be fine. There are quite a few different brands available these days. They're mostly intended for Axolotls or aquatic frogs, but they are perfect for aquatic newts as well if they'll eat them. My newts don't seem interested in pellets, but my Axolotls were fed a diet based on sinking salmon pellets without any issues at all.
I can't recommend any specific brand but price is normally a good indicator of quality, just steer clear of the cheap freeze dried stuff that's intended for fish or turtles, its nutritional value is normally pretty poor and newts struggle to digest most of the stuff that's in there. If you keep an eye on the classified section of the forum there are a few members who regularly sell good quality salmon pellets.
