Why is my goldfish turning black?

I bought a fantail goldfish about two months ago, and I am keeping him in a 10 gallon tank near no other fish. Recently, however, his scales rapidly turned black, and his fins. I took him back to the pet store, and they have no perception what is going on. He is acting completely normal, no red or white spots or things coming out of him. His slime coating is in great condition. My water is virtually fail-safe, and I have tested it numerous times, and I've done 2-3 times a week water change since I noticed his condition. What is wrong with him? Will he be okay? What can I do to fix this?
If you are worried you can get some broad Spectrum fish medicine. but he sounds fine. if he be sick his top fin would be retracted and he would be either laying on the bottom of the tank or swimming funny. Some times fish loose change colour depending on how much light they are getting... have you tried moving him?
it would be wise to upgrade to a 15g or 20g tank to give him some more room to grow. he most possible won't reach a foot since he's a fancy, he should get between 6" to 10" but a foot is not impossible.

I wouldn't worry just about the color change unless he starts acting different, as explained above.

just be sure to cycle the strange tank before adding him, contained by case you don't know it involves adding ammonia to the water an carrying out tests it and takes about a month... here's more on it if you inevitability it:
Answers:    While developing color in goldfish is adjectives black is not one of them.

His fins are turning black due to healing from ammonia poisoning. This can happen very suddenly and drop very quickly. It is a scary thought that the fish store didnot spot this right away

Stop doing 2-3 water change contained by a week. Return to the weekly 25% water change when cleaning. His color will come back as soon as he heal.

As long as he is swimming and eating with no other outward signs of problems, it was only just an ammonia spike.

Goldfish do change colors over time, as long as hes acting conventional and water parameters are okay he should be fine BUT a 10gal is too small for a goldfish, they average over a foot in length
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