Axolotls are actually a salamander that never developed out of it’s ‘tadpole’ stage, so they are fully aquatic. They breathe underwater, but they do have rudimentary lungs that can hold air. I’ve seen both of mine go to the surface, take a gulp of air, and then sink back down to the bottom again. They usually end up blowing bubbles, those silly little buggers. I currently have 3 adult axolotls, each between 6-7 inches long in a 20 gallon aquarium where they all live quite happily.