I bought a goldfish an i wonder how can i tell whats the gender?

Answers:
http://www.geocities.com/tokyo/4468/fish.
http://www.flippersandfins.net/goldfishf.
Bothe sites have good pictures
A

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Many fish are hermaphrodites -- which means that any one fish can make both sperm and eggs -- but goldfish are either male or female, just like people. Unfortunately, most of the time you can't tell the difference between the two sexes. The only time you can tell whether a goldfish is male or female is during their breeding season. This is usually in the spring, since goldfish breeding behavior is triggered when the water they live in gets warmer. When goldfish get ready to breed, the females become fatter as they fill with eggs to spawn, and the males develop tiny white spots on their head and gill covers.

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mature male goldfish will get bumps that look like acne over their gill plates at mating time. Male goldfish also have small barbs on their fins that can get caught in a fine white net. Goldfish live a very long time (some koi have lived over 100 years) and if you are looking to breed your goldfish you will need an older specimen (4 years old or so) and i would suggest you get a dozen or so and keep them in a very large aquarium.

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Males will often have Breeding tubercules develope on the gill plate and also on the leading edge of the pectoral fins. Sometimes these will stay permanetly, and others will go away again. But do not mistake them for ich.. The tubercules will be more uniform, while ich is like salt sprinkled all over helter skelter on a fish.

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all of these answers are correct but you should note that it's highly likely you will not be able to tell the difference in sex if you keep the fish indoors. Fish kept outside experience noticeable changes in their bodies with the seasons.