For this Goldfish, the "Gravimetric method" might be the best way to measure fecundity. Volumetric and Von Vayer methods are generally used for large sizes of fish eggs. For this goldfish, you may try to "Gravimetric method".
Remove the ovary filled with eggs. Weigh the entire ovary. Take 5-10 gram of ovary by cutting it. Count the number of eggs present in it and then multiply the same with total weight of ovary to get number of eggs present i.e fecundity