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Tail shape or length has nothing to do with goldfish gender. Its very difficult to sex goldfish until they have reached sexual maturity. Males will develop characteristic breeding tubercles on their gill plates and pectoral fins and have a more streamline build. Females may appear a bit more rounded or even lopsided when viewed from above. Venting is a more accurate way to determine gender as males will have a concave vent and females will have a convex (or protruding) vent. Heres a link with pics: