The multipurpose medication isnt any good for your axolotl malachite green and meth blue are poisonus to axolotls, even though you cant see it being bad for them it is really bad for them as i used it once and got told off on here for it, so in the future dont use the medicine as it is very toxic to them!
I would have only reccomended a salt bath but as other members have said not too Im not sure, ive had a similar looking thing on my axolotl used salt and it cleared it up, I dont know why its too big to be treated though... anyone know why?
Get the gravel out it doesnt help at all, fridging will slow it down but not cure it so follow which advice you would think of in respect to salt and tea baths
I know that feeding axies fish can be a huge problems as a lot of feeder fish carry terrible diseases as they are not bred well, so this could definitely be a cause.
So i'd say steps that will help right now are:
-Decide if you want him her in the fridge, it will slow the process of whatever he has down. It will help a lot but wont cure the axolotl
-Clear the gravel out and do some water changes keep it as clean as possible and monitor the temperature like a hawk, as if you decide you dont want him in the fridge then too hot water will speed this up a lot, and will become out of your control.
-Again if you did decide to keep in tank then monitor ammonia nitrite etc.
-Definitely stop using multipurpose medicine as it is bad for axolotls.
-Get a salt or tea bath on the go and monitor how this helps
- Id stay away from feeder fish and keep to earthworms or pellets or beef heart etc as they carry a lot of diseases.
If theres anything I've said which has contradicted anything on here then im not trying to, but this is what I would have done personally having poorly axies before!
Best of luck, i think temp or feeder fish have caused this but again may be completely wrong, but seen a lot of really bad axies with diseases similar to those in guppies and other feeder fish.
Sheena22