Salt

does anybody knows if it is ok to use this salt, it´s from rena it says that is
An all-natural salt, made from evaporated sea water.
Helps improve gill function to reduce stress.
Reduces electrolyte loss and promotes healthy gill function.

it´s this kind of rena´s salt
http://www.aquarian.com/cgi-bin/trans.pl/en/products/rena_range/care_vitamins.shtml

Joao, I don't really know about the Rena salt, it's derived from sea salt & may be just suitable for marine fish.
You could ask them, I guess? http://www.aquarian.com/cgi-bin/trans.pl/en/expertise/answers/submit_question.shtml
You seem to be having problems since you added the sand to the tank, is that right? Can you show us a picture of the sand, name of it, etc., please?
Have you seen the following?
http://www.indiana.edu/~axolotl/axolotls/shortguide/guide.html
Water
Water is the most important component of the axolotls' environment. Never house them in extremely soft or distilled water. They need hard water to help them maintain the integrity of their skin, their most important defense against infection. Remove any chlorine, chloramines, or ammonia that may have been added as part of municipal water treatment. Commercial preparations (e.g., Amquel) are available for this purpose. We pass our water through an exchange resin to remove heavy metals and past a sterilizing uv light as well. We also add salts to the water to make a modified Holtfreter's solution. The recipe we use to make 40% Holtfreter's in a 44 gallon barrel is:
KCl: 1 teaspoon
CaCl2: 2.5 teaspoons
MgSO4.7H2O: 2 tablespoons
NaCl: 240 ml (dry but measured in a liquid beaker)

More on salt solutions

The salts restore hardness after water treatment and help us maintain the animals' health by discouraging parasites and fungus. Extra salts are not essential, however, if you are attentive to good husbandry practices and the water is hard and free of chemicals and heavy metals. Keep the pH between about 6.5 and 8. If pH is at the high end of this range, monitor ammonia carefully because its toxicity will be greater than at neutral pH.
http://www.indiana.edu/~axolotl/axolotls/shortguide/solutions.html
100% Holtfreter's Solution & 100% Steinberg's Solution

I put some aquarium salt one a month in my axie tanks, but axies cant really be exposed to salt for a long period it burns their skin in high doses. Aquarium salt etc are good to reduce stress on fish.

where I work I never had problems with the water of the axies, I know the levels that the water should be, there are more than 10 in the same tank and they are fine , it´s here at home where I do everything that is done in there, that I´m having problems, I was wondering if it is from soft water, but I never tested it for it, I was only trying to find out if this salt was good ,to use if necessary, thanks.

I will try to get the picture of the sand.

Some salt can be added to harden up the water by adding calcium and magnesium salts. Sea salt is good for this. You just don't add enough to make the water 'salty', just a touch to give it some hardness.