There are two sure-fire ways of sexing Goldfish, even at 8 - 10 weeks of age.

1. The front ray on the Pectoral fin is thicker in the male fish than it is in the female. (Tubercles on a fish do not determine the sex of the fish 100%). I have had females that are covered in tubercles and have spawned every four to six days!!!!.

2. The easiest way is to look at the anus of the fish, in females it is convex (slightly protruding from the line of the fish), in males it is concave (slightly indented from the line of the fish). This method is 100% accurate although you might have to catch the fish and turn it upside down to check in a fast swimming active fish.

I will post some close up photos of fishes bums over the weekend for you to see exactly what I mean.

Hope this helps with any future breeding projects you may be contemplating.