Time to start reading up on "the cycle". :happy: Here is a quick overview.
Water chemistry is interesting.
Ammonia (produced by axolotls and decaying organic matter) is toxic, but the toxicity is dependent on water pH. High pH = ammonia more toxic. I suspect you have low pH.
Bacteria that eat ammonia will colonize the surfaces of your aquarium and decorations, etc. It takes a couple of weeks. Unfortunately, these bacteria eat the ammonia and then produce nitrite. Nitrite is also toxic.
Bacteria that eat nitrite will also colonize the aquarium, and it also takes a couple more weeks. They produce nitrate, which is less toxic. Too much nitrate will stress your axolotl, and is associated with fungus outbreaks.
The nitrate is removed by water changes. It is also removed by plants, but that doesn't eliminate water changes.
This growth of good bacteria is called "the cycle", and if they die for whatever reason, they say "the cycle crashed."
If you started with a large tank and a very small axolotl, your tank may have cycled without a huge ammonia or nitrite spike, but that is not typical.
Filters provide much more surface area for good bacteria to grow on and a steady flow of water to the bacteria. Some people can keep aquariums without any filter, but it is risky.
