Breeding Goldfish
Goldfish are seasonal spawners; they breed in the springtime. The females swell
with eggs, which attracts the males. The males develop tubercules (a sort of pimple
on the head) which makes sexing the fish easy in spring. In the other seasons you
can't tell them apart. Man has bred the goldfish into fancy shapes including the
round-
Breeding goldfish in the aquarium (when Spring comes) is difficult. They need the chase where several males buffet the female until she has the eggs squeezed from her. This takes a long swim, so breeders use ponds rather than aquaria. Fancy goldfish breeders hand spawn the fish by stripping eggs and milt from the ripe fish, but this takes knowledge and expertise. To learn more, UK readers can see 'Fancy Goldfish Culture' by Frank Orme about £8 from Nimrod Publications, 15 The Maltings, Turk St., Alton, Hants., GU34 1DZ (ref: ISBN 0 904558 63 0, 1979 Saiga Publishing).
Once the Goldfish have spawned the eggs and fry will be eaten, so the parents need to be removed or the eggs and fry moved to an aquarium. Note that raising fry in a tank is not easy because they will require feeding every few hours with tiny foods (infusoria, then freshly hatched Brine Shrimp, then crushed flake or commercial fry food). The problem is pollution; the water must be changed (partially) every day.
The other problem is runts...Nature expects most of the baby fish to be weak or deformed and they become food for predators, leaving the few good fish to escape. You have to be the predator, rigorously 'culling' the weaker fish. One brood will only give half a dozen sound fish and of these perhaps one or two will be good specimens for further breeding.
