I don't recommend salt for mollies (they need calcium hardness), but if the fish is hanging out at the bottom, add 1 teaspoon of salt per 10 gallon.
If the molly is swimming strangely allreaddy, take it out and keep it with methylene blue at the same concentration as above for 3 days, it will start looking better in hours, if I am correct.
Mollies are a good canary in the mine for a host of issues, they are sensitive to bad water, and black ones will show visible signs of external problems, like ich, long before you notice it on other fish.
They just have nasty personalities when healthy. I'm a fish guy, plants to me are just a means to an end, and their presence is more important than their appearance...but we have tonnes of guys on here for that.
Cape Town, South Africa.
Hi. I'm back.