Fish swimming with their top fins down

Hi all,

I was just wondering if anyone out there knew about fish swimming with
their top fins down. I have a tank full of goldfish, fantails and
black moores and they have started swimming with their fins down for
some reason. They don't look sick and they have lots of energy. They
are eating well and look healthy except they have started swimming
with their fins down.

If anyone has any suggestions why this is or knows what might be
causing it I would appreciate a note.

Thanks