Hi Lee,
If you are going to salt bath you should do it for 10 minutes once or twice a day, you should use 2-3 teaspoons of aquarium salt or non-iodized table salt in a litre of dechlorinated water in a 2 litre container. Don't leave the axolotl in for more than 10-15 minutes each time, because the salt will start to damage the axolotl's skin and gills.
Clean the container out with water and bi-carb soda before the next bath.
I would recommend using a net to remove the axi from the tank to the container than back again, as it is less stressful for them not to be handled.
Looks like you have caught it early so should fix your axi up quick.