Save my platy!! ;-;

Recently, my platy has been acting strange and looking really ill... it’s tail fin is clamped and may be ripped/rotting and it’s been sitting at the bottom of the tank, hardly swimming. Until now it was perfectly healthy. The picture attached is me introducing two of my fish into a larger tank, which is why they are in a bag. Please help me get it back to good health!!

Could be pregnant.Water parameters please?Does she eat well?

Unfortunately she died ;-; she stopped eating and couldn’t swim at all, she started rolling around trying to swim but just couldn’t. My water parameters are ammonia:0 nitrate:0(tiny amount) nitrite:0 . I noticed what looked like a faint green tint on her scales, possibly velvet but I did research and don’t know what happened. Do you have any idea if whatever happened could spread to the other fish/ how I can prevent it? Thanks

I would say it may be parasites.Internal,as to treat it I have no experience with parasites so do some googling or another member may be able to help