the smell will be from where the sand has been disturbed, when you have sand in an aquarium either there needs to be animals that burrow/sift through the sand or it has to be turned over by hand, this is because denitrifying bacteria can build up in the sand where there is a lack of oxygen, these bacteria break down nitrates but in the process produce nitrogen sulphide.
your axolotl looks very lethargic, for treatment used holtfreters solution and perhaps use methylene blue bath, holtfreters make the water more suitable for axolotls helps prevent fungus and helps protect against nitrites and nitrates, methylene blue as well being a dye and anti-bacterial acts as a medicine to reverse poisoning by nitrogen compounds.
for holtfreters go to
Axolotls - Requirements & Water Conditions in Captivity use at same amount as author of article using magnesium sulphide at 0.1g per litre same as author.
until ingredients can be brought use non- iodinised salt ie.. marine in the water at 2g per litre, this will help start recovery.
for baths go to
Tubbing and Safe Meds - Oregon Axolotls