Do you have anything to measure the temperature in your tank?
I use a water thermometer, it's in there 24/7. If you don't have a cooler/chiller of any kind, it's best to check that as often as you can. Large fluctuations in temperature seems to stress axolotls out a lot.
For example; it's better to keep your axies at a constant 20°C rather than switching between 20°C and 16°C.
Also, if your ammonia gets that high in spite of the fact that you do so many water changes, something must be wrong. Is your tank cycled or not? Do you scoop out poop and leftover food?
Also, what are all the other values of your water test? (pH, Nitrates, etc)
What kind of filter do you use, what is its stream strength and how large is your tank? (try and give the tank dimensions rather than liters)
Is your tank bare bottom or not?
Try and supply as much information as possible so the best help can be given.