Guppy with wart on tail fin

Question
My tank has been set up since August.  It is a 20 gallon
long with two guppies, 4 albino cories and one zebra danio.  
The filter is a Tetra Whisper 30.  pH is 7.8.  Ammonia is
at 0.  Nitrite 0.  Nitrate 10.  I do a 50% (10 gallon)
water change twice a week (last one was today).  

One of my guppies has developed a "bubble" or wart looking
thing on his tail.  It is the color of the rest of his tail
(bright orange) but very alarming looking.  He is acting
normal.  No lethargy or scratching or anything.  Just has
an odd growth on his tail.  Any idea what could cause this?

Answer
Hi Tim;

He may have been bitten by another fish or it could indeed be a wart or tumor or something. If it looks reddish or fuzzy like cotton, it could be an infected wound of some kind. If you could get a photo and send it as an attachment to a followup I could have a look...

At Your Service;
Chris Robbins