Axolotls frequently stop and tread water for ages, bobbing up and down in place, before moving off again. This is particularly true in shallow 1 or 2-block-deep water, but can also happen in deeper waters, such as in rivers. It happens when they come into contact with a solid block at the bottom of the body of water, they just abruptly stop swimming for 10-15 seconds. They can even get stuck in a nook because of this bug as they are unable to turn around.