"Aquarium salt" (Sodium chloride or NaCl) is one of the main ingredients in Holtfreter's Solution.
Holtrefeter's
Solution
NaCl
KCl
CaCl2
NaHCO3
Axolotls - Requirements & Water Conditions in Captivity
In a pinch, just the salt will do, (I use canning salt since it's the exact same thing as aquarium salt, just less expensive) although I also add a bit of baking soda (Sodium bicarbonate) to raise and buffer the pH and KH.
Here is the recipe for Holtfreter's Solution, if you're interested:
"Modified 40% Holtfreter's Solution: in 44 gallons H2O (Indiana University Axolotl Colony) - for use with embryos, larvae, and adult Axolotls (Ambystoma mexicanum).
KCl (Potassium Chloride) 1 teaspoon
CaCl2 (Calcium Chloride) 2.5 teaspoons
MgSO4 x 7H2O (Hydrated Magnesium Sulfate) 2 tablespoons
NaCl (Sodium Chloride) 240 cubic centimeters (dry volume)
If my calculations are right that would be 16 tablespoons of salt. Note this is to be added to distilled water..."
Holtfreter's Solution : Freshwater
Unless you're using distilled water, reverse osmosis water, or have extremely soft water from the tap (very low in GH and KH), I would recommend starting with 20%, then testing your mineral and pH levels to see where that gets you.