sick zebra danio

Question
I have a 20 gallon tank with three black mollies, five neon tetras and five zebra danios.  One of the danios is swimming strangely, almost vertically with his head tilted at a weird angle.  He will swim this way  for a few seconds, then stop and float for a moment, then start up again. He is thinner than the other fish and his body seems bent inward at the spine, forming a slight V shape.  Yesterday I looked into the tank and found him lying on the bottom.  He lay there for several minutes until he was bumped by another fish and then started swimming again, only even more erotically, almost like he had had a stroke.  Do you know what kind of disease would cause these symptoms?  All of the other fish look fine so far.  Should I isolate him? And then what? I've  been combing the internet for information, but I'm still pretty ignorant when it comes to fish ailments.  Any advice would be greatly appreciated!

Answer
Hi Erin;

It sounds like a case of a Mycobacterium infection. It is commonly called "Fish TB". Unfortunately he isn't going to survive much longer and if your other fish nip at his body they could become infected too. Once the spine becomes deformed and they can't swim properly the disease is quite advanced. Euthanizing him is the most humane thing to do but it is up to you. But please don't just flush him down the commode. It's a long slow and suffocating death. Here is a link to an article about humane methods of euthansia for fish if you choose that route;

http://www.wisegeek.com/what-is-the-most-humane-way-to-euthanize-a-fish.htm

Whatever you decide, wash your hands with soap and water thoroughly after coming in contact with the sick fish and sterilize your fish net or anything else that comes in contact with the sick one with hot water and a tiny bit of bleach when you are finished. (Just don't let bleach water touch your tank with the other fish) There have been very rare cases of localized skin infection in humans from this bacterium. Here is a very good article about it;

http://www.aquarticles.com/articles/management/Keefer_FishTB.html

At Your Service;
Chris Robbins