Treat for camallanus worms, they are really difficult to get rid of, you will need a really strong medication Sera Nematol is good it does work, I have used it, but needed a second dose. Once treated for worms you will also need an antibacterial medication to help with internal infection caused by the worms and to combat bacterial build up from dead worms rotting in the gut, feed peas 24-48 hours after the treatment to help the fish pass the dead worms. I have just had the little blighters in one of my tanks a rummy nose tetra went exactly the same as your platy, fine then bent spine as the poor thing was wasting away. Having just gone through it its 50/50 whether the platy makes it as the treatment can kill them, I lost 3 fish in total treating my tank. I would also treat any tank with fish that have been in contact with this platy.