Black spot on fish's side....

[riddle4sphinx]

Hey everyone, I have a new problem for you all. I hope I'm not just being paranoid.... I have a fantail goldfish in my cycled 20gallon tank. He's mostly orange with some black on the edges of his fins. I got him about two weeks ago and yesterday I noticed that he had a black spot on his left side. The spot is about the size the O makes on your keyboard, and really just looks like his scales are black. The ph, nitrate, and ammonia levels are all good, so I was just wondering if you guys thought I should worry, or if fish do change colors like this? I'm almost positive that the spot was not there 2 weeks ago. Thanks for any help!

[kcooley]

it may be the start of an infection, to prevent any further spread i would dose erythromycin at 200mg per 10G every 12 hours after a 45%waterchange. remove carbon for time. change 45% after 2 days/4 doses.

[fantail]

Blackening could also mean ammonia burns healing. As you now have the fish in clean water this might explain it as it takes a couple of weeks to show. The healing may get worse so keep an eye out and theres nothing you can do. If you want to google a pic then look for melanophore migration.