Adult goldfish will eat baby goldfish when they are placed in the same tank. Goldfish females will also eat the eggs as soon as the male has fertilized them, so it’s a good idea to remove her and the male from the tank once she has spawned and the eggs have been fertilized.
One way to tell when goldfish are ready to have babies is when the male begins chasing the female around the tank for hours at a time. The male will bump on the female’s abdomen, and when the female is ready, she will deposit the eggs. The eggs will stick to nearly anything, so it is a good idea to have a spawning mop, which will hide the eggs while the babies grow.