size is the key, of the fish and the tank. most successful way is to lay a broom handle over the top of a pond with knitting wool laid over it that trails in the water, males chase the females in a game of chase, as she passes through the wool she will lay her eggs, male chasing fertilizes them. i have had loads of success doing this in a pool 2m long, 1m wide and 1m deep but to get the fry big you need to remove the adults. my adults were commets and standard gold fish
in the uk we can't feed live, i breed for resale, but i found it was easier to buy bulk fry and raise them, koi/gold fish fry is cheap can normally pick up low grade fry but the thousand for not much out lay.