Irredescent catfish (Shark)

Question
They sold it as a Irredescent Shark, but then I seen the same fish in a different store and it was called a catfish, so I am not sure if its a shark or catfish.  My tank size is 29 gallons, I have had this tank for 4 years, and the Catfish/Shark I bought when I established my tank. I have a crab, 3 neon tetras, 2 head and tail lighters, 2 male guppies, 2 Silver dollars, 1 Plecostamus, 1 upside down catfish, 4 painted Tetras. I have had all my fish for at least a year, with no new ones added. Tonight I noticed that my Irredescent Catfish, has what looks like a blood blister on both sides of his tail in the same exact place, and there are some littles ones forming also. I have never seen this before on any fish I have owned.  Could you please help me to determine what this is, what might have caused it, and what I can do to clear it up if anything. This is also affecting the way his tail moves.

Answer
Yes, this is true many places call them irredescent sharks and some just catfish. They are really the same thing, just different names. Could the problem possibly be the crab? Maybe the crab is snapping at the shark? While the crab could not have been causing any problems for a while, as the fish get bigger and grow into their environment they tend to get more territorial. In fact, I recently had two bala sharks who normally should love to school together but as the bala's grew the tank size was causing problems. Soon, the "alpha" bala was trying to kill the weaker bala, causing me to have to remove the bully bala. I would suggest watching your fish and making sure no one is picking on anyone, particularly the carb picking on the shark. However, watch everyone as sometimes the least likely suspects are in fact the culprits. On the other hand, it could be a disease/parasite or maybe even a fungus. However, this is not very likely unless another fish has something and/or there are more signs. Check all the fish and make sure no one else has the same. Also, is everyone eating? Can they swim right, breath normal, and have normal fins? Do any fish have clamped fins? If any of these things is showing up as a sign, then it is likely a parasite/disease and should be treated. However, more investigation needs to be done. Take a look at your fish again and thier behavior and get back to me if you have anything more to add. Also, do not expect to see the crab picking on the shark as sometimes they only do it at night when its dark and the shark is easier to get. Sometimes, the crab will wait until the shark is almost sleeping and then snap at him. Just take a look and get back to me. I hope this helps. Get back to me with your results!