I'm sorry about your Platy, but I'm just as puzzled as you are. I was looking on my Aquari-Sol package (it's mainly an
ICH treatment, but it treats lots of other things too), and it said it treats a parasite that gives fish a grey-white film...does that sound anything like your platy? If so, you might try Aquari-Sol, just take out any snails because it kills them. However, this is a total guess. As for swimming upside down, she could have a swim bladder problem, in which case I'd stop feeding for a few days, or she could be dying

On the other hand, I know nothing about platys, so this could be normal for pregnant females. If I were you, I would isolate her, raise the temp. to 85 F if she can take it, and add at least 1 tablespoon of aquarium salt per 5 gal. as a general treatment in case she's sick. I'd also try Aquari-Sol, since it's mild enough that it won't hurt your fishie, even if it doesn't help (plus it's good to have on hand, since it treats a lot of diseases). I hope this helps, and I hope your Platy gets better
