Unhappy Fish :s`

I appologise if this is a common question but as ive never really experienced any diseases before im a little concerned. I have a group of 4 Serpea tetras and they are of quite a size now. However one has started to develop what is either a bacterial infection or fin rot....i have attached some pics so people can see...



What can i use to treat it? Or more to the point what is it? Will it infect other fish? any help would be greatly appreciated

whats the tank stats - ammonia, nitrite, nitrate, PH?

Any other fish showing any poor signs.

Looks fluffy to me in the pic is this correct if so its columnaris.
Can you answer some of the above questions.