Hi and welcome to the forum
How long has the tank been set up for?
How often do you do water changes and how much water do you change?
Do you gravel clean the substrate when you do a water change?
What sort of filter do you have?
How often do you clean the filter and how do you clean it?
Have you tested the water for ammonia, nitrite, nitrate, pH and GH (general hardness)?
If yes, what are the results in numbers?
Are the skinny fish still eating?
Female livebearers naturally lose weight (get skinny) after giving birth. If the fish are still eating then there shouldn't be a major issue. However, if there are complications when she is giving birth, she can die afterwards due to the complications.
Livebearers naturally occur in water with a pH above 7.0 and a GH above 200ppm for guppies, platies & swordtails. And a pH above 7.0 and a GH above 250ppm for mollies.
Make sure the general hardness is above 200ppm or the mollies will struggle to survive for any length of time.
If you ever lose a fish, you should do a 75% water change and gravel clean the substrate every day for a week. This will dilute any nutrients or disease organisms in the water that might have caused or contributed to the problem.
*NB* Make sure any new water is free of chlorine/ chloramine before it's added to the tank.
Clean the filter if it hasn't been done in the last 2 weeks. However, if the filter is less than 6 weeks old, do not clean it. Wash filter materials in a bucket of tank water. Do not replace filter media. If you have a filter that needs filter pads replaced every 2 weeks, let us know.
Add salt to the tank now and with every water change for the next couple of weeks.
You can add rock salt (often sold as aquarium salt), sea salt or swimming pool salt to the aquarium at the dose rate of 2 heaped tablespoon per 20 litres of water.
If you only have livebearers (guppies, platies, swordtails, mollies), goldfish or rainbowfish in the tank you can double that dose rate, so you would add 4 heaped tablespoons per 20 litres.
Keep the salt level like this for at least 2 weeks but no longer than 4 weeks otherwise kidney damage can occur. Kidney damage is more likely to occur in fish from soft water (tetras, Corydoras, angelfish, gouramis, loaches) that are exposed to high levels of salt for an extended period of time, and is not an issue with livebearers, rainbowfish or other salt tolerant species.
The salt will not affect the beneficial filter bacteria but the higher dose rate will affect some plants. The lower dose rate will not affect plants.
After you use salt and the fish have recovered, you do a 10% water change each day for a week. Then do a 20% water change each day for a week. Then you can do bigger water changes after that.