I've checked this with our care sheets, and 80L is enough for a fancy such as a black moor so with 106L you should be fine. In my experience with goldies the more important things are filtration and oxygenation than starting them in an absolutely enormous tank from day 1.
TBH, most LFS fancy goldies are so over bred they don't stand a chance of growing to their full size anyway.
You should be able to get both aquarium salt and methylene blue at any LFS and some garden centres stock them too. Non iodised rock salt can be substituted for aquarium salt and that can be bought at the supermarket.
I don't recommend 100% water changes as they can be stressful in themselves. Do you have tap water conditioner?
The tank size sounds like it should be fine, is it full to the top or only half way? If it's fuller the water quality is easier to maintain - and the axxie can swim about more!
If her tail is curling at the end that means she is stressed out and therefore more susceptible to diseases such as fungus.
How much water flow is there in the tank? Water flow can be a stresser as they naturally live at the bottom of lakes where there is very little water flow so consider turning the filter output down a bit or re-angling the output nozzle.
Lighting can stress them too, if you have lights consider leaving them off. I just have natural room lighting for my axxie and she's always bouncing and swimming around and stuff instead of hiding from the light.
Another thing that could stress them out is bad water quality. Have you got a water testing kit?
Sorting out the stresser will help him heal up much quicker along with treatment, help stop re-occurence and increase his lifespan greatly as well as giving you a much happier and more interesting pet
As for toys, mine has three brightly coloured plastic balls that were meant for a cat. They're always being moved about on the bottom - I must get a pic.
Salt water baths should help, I'd suggest a ten minute bath once a day for at least three days. 2-3 tea spoons of aquarium salt to 1 litre of water. Don't be tempted to leave him in for longer than ten minutes though as log exposure times can damage the gills.
What ever you do, don't use fish meds without checking with an expert as quite a few of them contain heavy metals which are poisonous to the axolotl.
Is the axxie's tail curling a little at the end? If it is it is a sure sign the axolotl is stressed out. Generally axxies only get sick when they are stressed.
What size is the tank and what filtration and water readings and temperature do you have? Also, Does the axolotl have any toys or things to interact with?
Re: Tank taken over by unwanted intruders! Help...
I'm just brainstorming here, but maybe the tiny white egg-like spots that cover the wood and plants, an no doubt the gravel (though it's harder to see) that I mentioned before are actually part of the white spot cycle,
Nope, protozoa are invisible to the naked eye whilst in their free swimming and egg stages.