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Goldfish can be difficult to sex even in ideal conditions. Do you know how old your fish are? How big are they? Breed?
At sexual maturity, goldfish can be vented. Males have a concave vent while females have a convex vent. Males will display breeding tubercles on their gill plates and pectoral fins and have a slimmer build (slim build does not apply to all breeds though). Females will appear rounder or even lopsided when viewed from above. Once again, this does not necessarily apply to all breeds.
Breeding behaviors can help to more specifically identify gender but this is not cut and dry as aggressive behaviors can be the result of numerous other issues. Basically, prior to sexual maturity, goldfish can be very difficult to sex. If you
have further questions, please do not hesitate to ask!