The more I learn the less advice I feel a hobbyist can lend with total confidence. Based on what you have posted here I would move out the five fish that are acting/looking normal leaving the single in QT.
What I have experienced here I'm confident identifying as camallanus worms. Heavily infected critters you can see the worms protruding from a fish anus, rather than a simple port (hole) or blunt tube it can appear shredded or even have live worms hanging out. Less effected fish looking only slightly bloated seem to react by getting a case of the fish poo's flushing out the dead bugs. Gill flukes have the fish 'puff' breathing after adding flu. (I'm not a sick fish know it all). Thankfully tanking now I rarely see stuff and am very happy to keep it that way.
I do lose a lot of fish during entry quarantines sometimes. (sadly)
Currently I have three small clown loaches in QT (started with six). Walking though looking before the purchase I saw no dead fish in any of the store tanks.
All six were very skinny when acquired but otherwise looked fine in the store.
Ich spots showed on two almost overnight in QT. Contacting the store to pass on the FYI I was told the store never
has Ich in it's tanks (LMAO). I just move on and expect it honestly. The remaining three are now fat active little piglets soon to be placed in a tank.
* When adding anything to a tank remember the directions referring to gallons or liters is in reference to a liquid volume not a marketed tank size.
I don't add much salt anymore and don't use the tree oils at all but offer this on salt.
A one-time addition of salt (1 level tsp/10g)
. Salt at this concentration (about 0.015% according to D. Walstad) hasn't damaged plants here and can help perk up a fin damaged fish without spiking TDS. (My entry QT is a planted one). To make sure the salt gets quickly mixed in the tank water I dissolve the salt in a container of water and then add the salt solution slowly pouring it into the tank.
Hope things go your way and best of luck to the fish.