Could it be a case of fin rot that just hasn't gotten to the other fish yet? My SAE lost his fins then later it seemed to spread to the tetras. The SAE later died but the tetras did not loose all of their fins and have since recovered somewhat.
I guess I would head to the FLS and see what they might suggest for fin rot. There is bacterial and fungal fin rot so try to examine the fins as close as possible and then read the packages or hopefully the LFS guy will know a little something.