How did you cycle it?
Was the molly in there whilst you were cycling?
One of the things about mollies (and platies, as well, as it happens) is that they don't do very well at all in soft water. Their bodies have evolved to take in minerals from the water, because that's where they naturally live. If your water in your tank doesn't have those minerals, it can, and does, cause health issues. THis is why Fluttermoth asked you about hard or soft water - since you don't know, it would be really good if you could find out.
For instance, does your kettle fur up with limescale?