Fin Nipping

[Connor hewitt]

I got some serpae tetras about 4 days ago and I have noticed that their fins are damaged and I don't know what would nip them as I only have peaceful fish in my aquarium.
I have Molly's, neon tetra, swordtails, gobys, scissortail rasborss, harlequin rasborss, albino bristle nose, albino corys, Amano shrimp and some dwarf clawed african frogs.
Does anyone know what could be fin nipping my serpae tetras, thx

 

[OrionGirl]

The tetras could be doing it....or something else. How big is this tank?

 

[the loach]

You have most probably less than 6, which is not enough that's why they are doing it.