Koi with Pimple like spot on fin

I purchased several new koi a couple of weeks ago and have them in quarantine. In the last few days one of them has developed a small pimple like bump on his fin. He doesn't seem to be bothered by it, swimming and eating fine.
I've done some research and found the this can be symptom of Anchor worm but at this point I see nothing coming out of the bump to indicate anchor worm. I did a water change three days ago but Ammonia and Nitrate are not at zero. I am working on this!

Could this be the start of an ulcer?

Any help, advice is GREATLY appreciated.

I can't post a picture because

Sounds like Carp pox, has been know for 400 years, yet not often recognized as a viral disease by koi pond owners. It is caused by a virus belonging to the herpes group, it is considered chronic but is not life threatening. There's no apparent treatment and the causes have been pinpointed on water quality and overcrowding, the lessions often disappear as water quality improves and the temperature increases, Happy New Year to all and wishing you a great 2012.

Thank you for the response!