After having got over a problem in which all of my white mollies were
too buoyant, I now have the complete opposite problem with one of those
mollies. This particular molly has no buoyancy whatsoever and has to
battle gravity to reach the water surface. She has lost her well rounded
figure to the point that she looks underfed, although she is eating
whatever is given to her. It's as if the swimming bladder has been
damaged during the period of her being over buoyant.

I presume nothing can "fix" the problem. I've never been able to cure
internal physical problems before. At the moment she can do nothing but
rest on the substrate when she's worn out of swimming.

Nikki