07-18-2010, 07:32 PM
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It looks like a natural change of color to me. If he isn't acting sick, I'm sure it's nothing to worry about. If if was red instead of black, it would be something to worry about.
BTW, your water looks cloudy in this photo, like there's a bacteria bloom. Are your ammonia and nitrite at zero? If not, you should look at how your filter is set up, how you're cleaning it, and if that's ok, then you might need more filtration. Plus, this fish looks quite fat (for a single-tail.) You could probably cut down the feeding to help with waste build-up.
Hope that helps!