Hi Natalie, welcome to the community, sad to hear it was because of this little guy's situation, but glad you are here.
I would add Epsom salts. It doesn't seem to ever cause more problem for the fish, but does often help them.
This is cut and paste from a recent post but still applies.
I don't know what size the QT is but the stronger amount here is 2 tablespoons for each gallon of water, and you can calculate what you would need.
Excerpt
"What I would do for my own fish...
Normal large
pwc.
I would also add Epsom salts magnesium sulfate, like for soaking a sprained ankle - available at the drug store or super market. No additives no oils, etc.
For the 2.5 G I would take 5 rounded tablespoons and mix it into an adequate amount like a cup, of really hot water til dissolved. Then add it to appx. double amount of cold water or ice cubes until tank temp, and add dechlorinator.
Remove the same amount of water as what you have mixed up and then over the course of the next day, add it to the tank.
This is a treatment amount of epsom salt.
initially add in 1/4 of the mixture, in two parts.
say half /part of the 1/4 amount then about a half hour later, the other half.
This just eases the water change for the fish.
In 8-12 hours... add another 2 part 1/4 amount of ES over a half to an hour.
Continuing until it is empty.
You can test for any water parameter issues during this time. I would keep up this amount of ES for 7-14 days at this increased level.
You can take more time later to make a concentrate of amount of water for amount of tablespoons of dissolved ES so it would be easy to dose it in for each
pwc. So the amount is per each .5-1G
pwc. As a fixed concentrate amount. Hopefully it will be easy for you to figure that out. I just keep it in a water bottle marked with a Sharpie.
After the 2 weeks if he is feeling good and acting good, I would do the normal
pwc and decrease the concentrate added to equal 1 Tablespoon per gallon. Try that for a week or 2 if he seems good, then just do normal
pwc and over time the excess ES will decrease and be eliminated."
I have had good success with this treating Betta. One fish had been so ill that when I reduced out the ES at the end of the treatment he would get ill again and he never ended up being 100% but lived a good life for another year after, and I just kept the 1 tablespoon per gallon in his water because when I didn't he would be acting ill again. He needed continued basically perfect water.
Other fish were just fine after treatment and didn't need it continued.
If you are feeling like he is on the verge of death you can just add the initial ES concentrate in a little bit every 15 min to half hour when you are awake (as in do not stay up all night to dose it- get some sleep), to get it in more quickly.
I have actually added it in small amounts when I felt that the fish was near death and all within a 2 -3 hour period, and did not seem to have an negative effect on the fish. The excerpt quoted initially would be a regular treatment with average urgency/unwell fish.
New to FW fish? First tank in a long time or EVER??? Need some info on keeping fish? 
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