Our Male Guppy
Salt is good for "Livebearers". No more than a teaspoon for every 5 gallons of replacment water is all that's needed. Most pathogens don't tolerate even a trace of salt in the tank water. More than a teaspoon and your aquatic plants won't do well.
I don't quarantine my fish and don't think it's needed, too stressful. So, dosing chemicals isn't an option. They can affect your other fish, plants and sometimes kill your beneficial bacteria.
I suggest an aggressive water change routine for the next few weeks. Change half the water every week, no slacking. Make the new water a bit warmer than the water that's in the tank and dose one teaspoon of salt for every 5 gallons of new water. Get some Anacharis (Waterweed) and float it in the tank. I find this fast growing, leafy plant helps calm my Guppies. Good nutrition is important. I feed mostly frozen, because there are no additives. Feed frozen brine and mysis shrimp, plankton, beefheart and krill. Just a little every couple of days. I also feed a bit of minced garlic. It's a natural antibiotic, with a high vitamin content.
The combination of a lot of pure, treated water flushed through the tank, a little heat, salt and good food will generally help.
Just one reporter's opinion, though.
"Fear not my child, just change the tank water."