I took this right off of axolotl.org
"Axolotls prefer somewhat hard water, and those that live in soft water will often suffer from temporary anaemia - the animal becomes pale and its gills lose their colouration for a few minutes or hours. This is not a dangerous condition but it can be prevented by supplementing the hardness of the water with added salts."
I use about one tablespoon of salt water fish salt per gallon of water. You can also use aquarium salt, rock salt, noniodized salt, or Hoftretters solution.