Does my Platy look sick or subject to "fin nip?

[Paul1792]

I took a minute to admire the healthy, happy, vibrant fish in the crystal clear polished water of my 38 gallon this morning and I saw something unusual I had never noticed. A female metallic blue wagtail (platy) appears to have some sort of what looks like rotting tail fin ... but with no sign of disease. Occasionally, she doesn't keep her top (dorsal? fin in the full "upright" position.

I have a feeling, however, that my Long Fin Red Minor Tetras (Hyphessobrycon serpae) could be the culprit since I have never noticed this before.

I don't know how long it's been this way ..... but I cannot imagine missing it before.

Complete stocking etc. information about my 38 gallon is in my profile.
API Master test kit results 3 days ago were ph 7.2, nitrite 0, nitrate 5 ppm.

I just took a picture of her which I posted here:

Aquarium Advice - Aquarium Forum Community - Paul1792's Album: Paul1792's 38 gallon - Picture

(upper right corner to the right of the red wagtail)

What do you think? Everything else in the tank looks perfect. Fish colors are excellent.

If disease. What should I do ?? I don't have a quarantine tank. Should I be using something like seachem flourish as a precaution?

Thanks in advance.