I've never seen it that bad, but when I have seen something like that, it has been due to a high bio-load. A friend of mine described something like that recently when he was doing a fishess cycle - i.e., lots of ammonia, which is a lot like a high bio-load.
Did something die in the tank? Did food get spilled in there? Has someone else been feeding the axolotls for you?
I recommend checking the ammonia, nitrites, and nitrates, and taking corrective action if necessary.