Why are all your goldfish chasing 1 particular goldfish?

Answer 1

Goldies are typically not agressive. The most aggressive behaviour is during spawning. Males will chase a female and bump her belly to cause her to release her eggs. Goldfish are egg scatterers which means the female drops her eggs all over the place as she swims along, she does not lay them in one place. The male then swims behind her and fertilizes them.

Sometimes if you have several males and one female ready to lay eggs they can get quite brutal and stress her from chasing. If you can move her to another tank or I have found the small plastic storage bins (with holes....looks like a small laundry basket) works great! Just float it in the tank and put her in it to let her rest if she is stressing or her tail is ragged from the chasing.