Fin Question?

Fins can and do regrow, but often not quite s they used to be. With short tail fins this is hardly noticeable.
Does the fin end at a clean well defined edge or is it ragged, maybe with white, black or red edges? The former is more likely to by physical damage, the latter to be fin rot. If it is just physical damage, a few extra water changes to keep the water nice and clean should be enough to stop the damaged area becoming infected.

Since you say the fish are new it could be old damage from the shop tank, possibly even while it was being netted out of the tank, or another fish in there took a bit out of it.

That's debatable. Most cases of finrot can be traced back to poor water conditions. These stress the fish, which lowers the immune system making the fish more susceptible to anything in the water. All the fish will be subject to the same poor water and although some individuals will be more resilient than others, some tank mates may also come down with finrot. Is this because the bug is infectious or because several fish have compromised immune systems?
This is another reason for water changes if you even suspect finrot - they improves the water condtions, and improve fish immunity.

10% a day should be fine; if you'd been doing 10% a week, I'd have suggested more.

Ok thanks for that fcmf. Have taken note.
The Panda Cory in question is still active, feeding well and generally going about its business. I still cannot see any regrowth of the rear fin but neither can I see any red or black marks that would indicate fin rot. Other Corys that came with this lil guy all seem ok as well. However will be monitoring it for the foreseable and am doing a strict 10% water change every day. Water quality is really good atm...no Ammonia, Nitrite and Nitrate has gone right down to about 10-20 (strange as my tapwater seems to be just over 30ppm?? Must be filter now reducing nitrates?)
Thanks for advice anyway 

"Re your nitrate -
This test is the most inaccurate as it is difficult to make an accurate one suitable for home use. And do you have live plants in the tank? They use nitrate as fertiliser, and will use ammonia in preference so the ammonia made by the fish doesn't get as far a nitrate"

Yes i have 2 small pieces of driftwood holding Java Ferns..growing nicely so i guess this is why ammonia is always zero and maybe why nitrate has dropped?