There are many reasons for an over active axolotl. Now ammonia is only a small part of your water chemistry and should stay low with your nitrate and nitrite levels. If you are only testing for ammonia I really recommend you buy test strips that test for all water parameters. Once you have your tank cycled and established you won't have to test it all the time like you're paranoid. Also doing water changes weekly at about 25 or 30% is a good way to ensure that you're not allowing toxins such as ammonia nitrate or nitrite to build up in your tank. But honestly water quality probably isn't the reason why he's moving around so much. Didn't you say you just recently got him? Maybe this is just his attitude and behavior to his new surroundings. Also what are you feeding him and how often are you feeding him it? And lastly do you have a pretty good idea of exactly how old this Axolotl is, juveniles do crazy stuff all the time. My female likes to swim back and forth back and forth through the bubbles coming from my AirStone. Just keep monitoring him and make sure it doesn't seem like he's stressed out and he keeps eating, there is a good chance that he's just being silly.