mixing species can be tricky. with most animals the rule is "if it fits in my mouth, its food". im guessing your bullfrog keeps his head above water most of the time whereas the newt is always under the water and out of his sight, right? so he hasnt eaten him because he is not hunting for things under the water. how much land is there? how deep is the water? the problem is that another animal will have to stay bigger then the bullfrog to not be eaten by it. hwo big is the bullfrog, in inches from nose to rump?what are the bullfrog's eating habits? does he just jump onto the land and grab all the crickets? what are you feeding him? how is the land setup? im not trying to find flaws in your setup, i am trying to figure out if there is something that i think would be a safe bet.