I'm sorry to hear things are not going well. I'm first going to be a bit defensive of adding salt. The basis of the recommendation is that it reduces the difference between the axolotls blood composition and the water and allows the animal to maintain its body chemistry more easily. You can have too much, but 1.5g per litre is well below blood level which is roughly equivalent to 8g per litre and the osmotic pressure of blood is higher, almost 9g per litre.
Higher levels have been tolerated experimentally by healthy axolotls, but they showed accumulation of urea and increased electrolytes abstract reference:
Adaptation of the axolotl (Ambystoma mexicanum) to a hyperosmotic medium
M.P. Ireland and I.M. Simons
Department of Zoology, University College of Wales. Aberystwyth, Wales, U.K.
Abstract
1. 1. Adaptation of the axolotl to a 30% sea water medium for 5 and 35 days resulted in an increase in blood electrolytes and urea.2. 2. After both periods of sea water adaptation the blood osmotic concentration did not exceed that of the external environment and the major ions contributing to the ionic concentration were Na and Cl.
3. 3. After 5 days in 30% sea water there was a reduction in water uptake and urine flow.
4. 4. A reduction in the size of the cells of the pars intermedia of the pituitary occurred after both periods of sea water adaptation.
5. 5. The results are compared with the aquatic anuran Xenopus laevis subjected to similar stimuli.
30% seawater is about 10.5g per litre. There is a wide margin here.
The human analogy works here. If I get fresh water in my eyes it stings a bit. Sea water is a lot worse but the 9g per litre contact lens solution is fine.
The white spot organism by contrast cannot reproduce in over 1g per litre salt. I think you should maintain the salt level even if you fridge, I did with an injured axolotl.
Despite this if things are not going well fridging is an option. It reduces appetite, the gills are often white when in the fridge, but an axolotl can do a long time in the fridge and it often aids recovery.
It could be a different disease. Tetrahymena Corlissi sometimes called guppy killer disease can affect amphibians and is systemic as well as causing surface white spots. It is fairly salt tolerant and I am not aware of an effective treatment. Again fridging may help. It is generally a fairly warm water parasite and in the fridge the axolotls immunity may get on top of it.
Dealing with the query how I manage my tanks. I generally do a 1% daily water change with the home made salt mix. If I am away and on my return the water level is down I top up with plain water and then go back to 1% changes. My tanks are a bit unusual, nearly solid elodea and watercress with a muddy bottom and no filter. I never clean the base and merely scrape the front face occasionally to try and see the axolotls a bit better! The plants mop up ammonia and nitrite and nitrate are almost always nil. I occasionally use an iron based fertiliser if the watercress looks yellow and stops growing.
I am sorry adding salt has not helped and think fridging may be wise. It may be best to keep the salt level the same even if you do fridge.
