New guppy sick. How do I help?

I got a lovely girl guppy from the lfs almost two weeks ago. At the time of purchase, she was slightly lethargic and clamped, very thin. Once home, I noticed she had a few scales sticking up here and there but was not pineconing.

I have her in a cycled 5gal QT (0a, 0ni, 15na). With food and quiet she perked up slightly but didn’t gain weight. I treated her gill flukes with prazipro 4-5 days ago. This morning, she looks markedly worse. First four photos are today. Last pic is from June 30.

Two other girls in QT with her aren’t showing symptoms. Would appreciate advice to keep them healthy!

Symptoms:
-rapid breathing
-lethargy
-ragged-looking scales
-emaciation

I’m not new to fish but new to guppies. What can I do?

(Photos in comments)