Illness/Sickness: Axolotl covered in fungus after fridging

For about a week now I've had my two adult axolotls in the fridge after an accident with the 20g required them to leave. They were doing well at first, but on Thursday night I noticed a bit of fungus on one of them. Friday I went out to get the salt and when I came back he had it everywhere. I began giving him salt baths using 2 teaspoons of salt per liter of refrigerated water, and leaving him in it for 15 minutes exactly. The fungus has not only increased, it seems to do so exponentially within hours. He is now largely covered in fungus and mostly unresponsive. As of today's salt baths, I have increased the amount of salt to 3 teaspoons per liter of water and considering lengthening the bath time to 20 minutes.

He's just getting worse and worse and I don't know what to do. I don't even understand how fungus could even grow in the fridge at all, let alone expand so much within small amounts of time. He's now mostly limp and it absolutely kills me to see him like this. The other one I put in the fridge with him is fine, and I don't understand why. What can I do to fix this? Is there anything left I can do?

He is no longer responsive. The fungus has spread even further and has largely taken his eye. Not quite dead yet as he still moves very slightly after long intervals, but it's quite clear he's well on his way out. I'm skipping his last salt bath so that he may pass more peacefully.

I feel so miserable because his final days must have been so painful. I really wish I could have given him more. The other lotl in the fridge is looking very well, but I'll be watching him carefully for the next few days.

Does anyone know what could have caused this? I've never heard of fungus spreading so quickly and causing so much damage within a short time.