Axolotls love bubbles and air stones, they often go and land right on top of the air bubbles, bubblers help oxygenate the water and axolotls also use them to gulp a little air, this helps them float better when they want to surface. You should use a quiet flow Air Pump with an air stone or a penn-plax airline tubing. Don't put it in the middle of the tank, it belongs on the other side where there is no hide.