Mollies need different water to the other fishes you have so this might be a blessing in disguise.
Livebearers like mollies need water with a general hardness (GH) above 250ppm and a pH above 7.0. The other fishes you have come from soft water with a GH less than 150ppm and a pH below 7.0.
If you have 2 tanks perhaps set one up with hard water for mollies and the other can have soft water for the tetras and barbs.
If you feed them more often they will be less likely to eat each other. In an aquarium with an established biological filter, you can feed the fish 2-3 times per day. You need to monitor water quality for ammonia, nitrite and nitrate but there shouldn't be any issues with ammonia or nitrite. And nitrates can be reduced by doing 75% water changes and gravel cleaning the substrate. Try to keep nitrates under 20ppm.
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If you want to add something to help you can add rock salt (often sold as aquarium salt), sea salt or swimming pool salt to the aquarium at the dose rate of 1 heaped tablespoon per 20 litres of water. If there is no improvement after 48 hours you can double that dose rate so there is 2 heaped tablespoons of salt per 20 litres.
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 2 heaped tablespoons per 20 litres and if there is no improvement after 48 hours, then increase it so there is a total of 4 heaped tablespoons of salt 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.
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If the fish gets white fluffy bits on the damaged areas or looks sick and stops feeding, post a picture of it asap and we can look into other things. But if the fish is in clean water then its fins should heal up without any need for medication.
I’ve had this tank 4.5 months and never had any issues.
