There are literally hundreds of choices of species to choose from, though for an aquatic terrain environment, only newts are adapted to living underwater and they do need some sort of floating landmass.. Some popular species are eastern newts, california newts, japanese fire-bellied newts, and their subspecies and many other species, you just have to do some research.
The only thing you must consider are the species of fish you have inb the tank. some larger species will actually eat newts, while some smaller agressive species may harrass newts. I have had some problems in which platies and swordtails forced me to separate them from my california newt. Though right now, with far fewer and more mature fish in the tank, they now get along. Some fish have there personalities, so just keep an eye on them. But I wouldn't take the risk with large fish, fish with teeth, or highly aggressive fish.