My female platy has been bullied and became the center of attention by my mickey mouse male. She has been hanging out around the heater and looked skinny.

I just pulled her to get a better look at her.
and found this on her back. What is it?!

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Image - TinyPic - Free Image Hosting, Photo Sharing & Video Hosting



If it's something do I need to treat tank?
We came to the conclusion yesterday I have soft water if that helps diagnosing in any way. I used an API 5 in 1 dip (not very trustworthy I know but it's what I have on hand)
GH 30
KH 40