Is the back end of the axolotl floating like that a lot, like in the last picture you have? It could be constipation in which case you want to fridge. -5 is too cold, are you able to put it up to around 5-10 degrees for at least a day? Or do you have a different fridge you can use? The food, etc, in your fridge should be fine at 10 degrees (I just checked my fridge and it is 10 degrees, we have a separate section that is set to -5 for freezing things).
What temperature is the tank? Have you noticed if he has pooped in the last week?