Hospital tank for ADF + enclosure questions

I'm pretty sure my African Dwarf Frog has a fungal infection. I moved him to my only other option for a tank (about 1 gallon, no filter or heater). I have two perfectly healthy ADFs remaining in the primary tank and added a little bit of salt last week and some maroxy a few days ago.

Gomer, the frog in the hospital tank, seems to be really unhappy in it, and is still fairly energetic. He looks ill (discoloration patches on his back, with either shedding skin or something like it hanging off him, his eyes are also moderately dark), but otherwise he acts fine. Very full of life, swimming about like the others. I used some of the treated water from the main tank to fill the hospital one, and I added a drop of maroxy to it today.

I am fairly new to all this, and wonder what to do. Is the hospital tank too crumby? Should I put him back in with the other frogs in the primary (as I said before, it now has salt and was twice treated with maroxy)? I was indecisive for almost a week about this and only quarantined him yesterday. I don't want to infect the healthy frogs, nor do I want to hand down a death sentence to Gomer.

I read somewhere that aquarium salt prevents bacteria and fungus from living/growing.

The primary 10 gallon tank currently has roughly 8 healthy plants, 3 black skirt tetras, 2 african dwarf frogs, an S15 marina filter, and a heated temperature of about 75F. I do a water change once a week, anywhere from 15-25%.

Suggestions?