Illness/Sickness: Everything wrong at once!

I have two male axolotl, who are almost fully grown. The smaller of the two has always been a bit slower to react compared to the bigger one, and the big one would push him out the way to get to food, but until recently they were getting on fine, and the little one was pretty much the same size as the big one.

Then I got home one night about two weeks ago to find the little one missing a foot and very stressed. It's obvious what happened there, so I think when the little one is returned to the tank, I'll have to put a divider in.

At first he seemed unfazed by it, so I kept an eye on them but left him in the tank. The wounded leg is healing over well but after a while he's stopped eating, has lost some weight, looks quite red and blotchy, and possibly has a small amount of fungus on his body and tail. At the moment he's been in the fridge for two days, and some of the fungus seems to be coming away with the water changes. I'm wary of doing salt baths because of how red his skin is already. I've read that API Pimafix can be helpful and is safe for axolotl, so was thinking of buying some. Might this help? Should I keep him in the fridge? Or would it be better to set up a hospital tank? I do have a spare tank, but it would take time to cycle. It'd be great to get some advice about which problem to treat first and how!

The big one, in the meantime, is totally fine, and eating normally.

Thanks guys!

Here's some pictures of him that I took today. (He's always had an upside down-looking back leg since I was given him, I don't know why.)