I don't think goldfish breed until they are big and mighty. And you cannot tell whether one is male or female until breeding time arrives. Depending on the size of the fish. If you know you have a breeding pair of goldfish, they are moderately large and followed FN's tips, you shouldn't go wrong. But if you really want to get alot of feeder goldfish for your oscar then a pond can be made too. Making a pond basically guarantees breeding without much effort. But if you don't need much, or that badly, then just the tank way is fine.