Serpae Tetra Cottonmouth?

[JakeSpell]

I was visiting my aunt's house the other day when to look at an old aquarium that my dad gave her and noticed she had one fish in it, a serpae tetra. It looks like he has cottonmouth on his mouth and sides near the head. When I asked about the tank parameters she had no idea what they were or even what the nitrogen cycle was... Apparently was just waiting on the fish to die so with a little convincing I got her to let me treat this little guy.

Details...
55 Freshwater set up for about 9 months
pH below 6.0
Ammonia .25ppm
Nitrite 0ppm
Nitrate >120ppm (not sure how hes still alive...)

What Iv done...
Moved into a small clean 10 gallon using the same filter from the 55
Used about 50/50 mix of fresh water to old tank water (tap water water has a pH of about 6.8, no idea how it got so acidic.)
Raised the temp to ~80
Added salt to the hospital tank
Added StressCoat
Added MicrobeLift (I used it to successfully treat cottonmouth in my tank)

What my plan is...

Go back and do partial water changes to try and get the pH up to the same as the tap and get the temp up to about 85 and continue to treat.

Does this guy have a chance or is this a wasted effort?

If he does improve Ill add him to my tank but how long should I wait to make sure hes healthy and not carrying anything harmful?

If the main 55 stays fish-less will any parasites in it die off after a few weeks?


Thanks in advance for any advice!

 

[JakeSpell]

Update:

After a few days it appears that all the fungus/cotton mouth is gone. I'v keep with water changes every other day, about %20 since the ph was so low.

My question is how long should I keep him in the qt and should I add him to my tank by himself or should I add two new ones with him? I have 3 at the moment and want 6