Help! Shubukin goldfish sudden problems

[rocket_pony22]

I have a 2-3 year old shubukin who started having slightly labored breathing and swim bladder/floating problems, these floating problem got significantly worse overnight while tank is being treated with med trio. I've just moved him to 5 gallon fresh clean water with a bubbler. There are no other outword signs of problems with this fish, however something is going on clearly in my main tank.

Quick background:  woke up yesterday to my smaller  1 year old sarasa dead, who also had not been showing signs of distress.  2 other older shubukins in tank have some minor fungus issues on their tail fins I've been battling off and on (they currently have a little on their fins) and I did notice one of them showing flashing/irritation behavior. Nothing has been added to this tank in over a year. It's a 75 gallon planted, over filtered 2 hobs.  The parameters at time of the sarasa dying where essentially ph 7.6, ammonia background, nitrite zero, nitrates 80 ( a bit high, was already setup for a water change).  I did a 60-70% water change twice yesterday, 12 hours apart.  Parameters are now same with nitrates around 10 ish.  I started the med trio recommended dose for quarantining/when you don't know what's wrong for the main tank after the second water change because I was worried, intending to let that one dose of meds sit for a week, but have now moved the distressed fish.

Im scared I'm going to lose another fish. He will swim around some, but I've put some things in this quarantine container so he can wedge himself and not be fighting to swim/expend energy from the flow of the bubbler. Hes wedging himself in upside down which is a bad sign. Do I treat this quarantine setup with meds? If so just erythromycin? Is there anything else I can do?

[rocket_pony22]

Just added level 1 (1 tbsp per 3 gallons) of aquarium salt in my quarantine container per the blog, 1.5 tbsp for estimated 4.5 gallon of water, might up it to level 2 if he's still alive this afternoon/evening. Other note, I live in Texas so our water is like liquid rock as far as tds.