Hi and welcome to the forum
Livebearers like guppies, platies, swordtails and mollies are fine with salt. If they started acting weird after doing a water change, there might be something in the new water that is causing problems.
Did you dechlorinate the new water before you added it to the tank?
All new water should be free of chlorine/ chloramine before it's added to the tank.
You can add some Activate Carbon or Highly Activated Carbon to the filter. If there is any chemicals in the water, the carbon should remove it. Remove the carbon after a week.
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What are the tank dimensions (length x width x height)?
What is the GH (general hardness), KH (carbonate hardness) and pH of your water supply. This information can usually be obtained from your water supply company's website or by telephoning them. If they can't help you, take a glass full of tap water to the local pet shop and get them to test it for you. Write the results down (in numbers) when they do the tests. And ask them what the results are in (eg: ppm, dGH, or something else).
Livebearers need water with a pH above 7.0 and a GH above 200ppm, (250ppm for mollies).
The other fishes you have come from soft water with a pH below 7.0 and a GH below 150ppm.
If the GH is too soft, the livebearers will have issues. If the GH is too hard, the other fishes can have issues.