My Pleco is sick

Question
Hi. My mom recently got a normal pleco from her friend to put in her tank (18 gallon I think), and it's about 6-7 inches long I'd say. A short time after we got the fish, it started developing a nasty-looking reddish-brownish spot on its belly which has gotten bigger and bigger. Now it's probably almost a centimeter in diameter. It looks like maybe even a small piece of skin has come off as well. I don't have a microscope or anything for further examination. What do you think it may be and how should I treat it? Should I quarantine it?
-Thanks
ps: I have taken a couple of pretty good pictures of where it is hurt. I posted them online at the following links:
http://i54.photobucket.com/albums/g106/MarineJim88/plecosmall2.jpg
and
http://i54.photobucket.com/albums/g106/MarineJim88/plecosmall.jpg
if you really need me to, I can send you some photos in much higher resolution too.

How long has your tank been set up? for a while
How big is your tank? 18 gallons
What type of fish? pleco w/other tropical fish (they seem to leave him alone from what I've seen)
How many fish do you have? probably (or definitly) too many for the tank, but my mom is stubborn
Type of filter? regular coal w/ bio i think
What are the pH, ammonia, nitrite levels? not sure
How often do you change water? not sure
How many gallons of water are replaced every time? n/a

Answer
Hi Jimmy;

Those are excellent photos! It's hard to get good clear shots of fish. Most of them I get are so blurry I can't tell what kind of fish they are, much less what the illness looks like. Good Job!

It looks like he was injured. The skin actually appears intact on most of the reddish round area, it's just hemorrhaging or bruised under the skin. I see these kinds of injuries when fish are transported with other fish and they get punctured by the spines or fins of other fish. Plecos and other catfish have very sharp spines on the ends of their fins and they can easily injure each other's soft bellies. That's what it looks like to me anyway. If there wasn't another fish moved with him, perhaps he got close to another as he was being netted and got jabbed. Or, one of your mom's fish maybe nailed him trying to chase him from it's territory as he was a newbie trying to find a spot to hide.

It will probably heal on it's own but if it becomes worse he may need medication. It's not a good idea to add medicine to an established tank. It can make a serious mess in there. A hospital tank would be needed. Just a 10 gallon with no gravel, no decorations, no filter. Those all absorb medicine and it can't work very well that way. Just lights, a heater and bubbles. Change 50% of the hospital tank water every day to keep it healthy during treatment and use medication such as Maracyn Two. The hospital tank doesn't have to be kept filled with water all the time but they are good to keep handy in storage for times like this.

Good luck and I hope he doesn't get worse...

At Your Service;
Chris Robbins