Goldfish tail showing spots

It may be ich, look out for more spots. Most healthy fish in a good environment will be able to fight it off on their own (I'm speaking from much personal experience with it) and it shouldn't spread. If it does start to spread and you begin to see little white spots (like rock salt) on the body then begin ich treatment (I'd say hold off for at least 48 hours, if it hasn't spread, keep an eye on it until it goes away but no treatment would be needed, which would stress the fish less). If treatment is needed I've always used raised temp and salt treatment, it stresses your fish and ecosystem in the tank a lot less than nasty chemicals. Hopefully you won't need any treatment though. Good luck! Wait for other opinions too, just to see if I'm even right...

No idea, but most of my white goldfish have one or two. It doesn't look like ich to me. One of my five year old all-white fantails has this particular marking. I'm not sure that is isn't just natural or a scar tissue on the fish.