Selecting pairs of fish

There is some discussion of how to tell the sex of goldfish elsewhere on this site. A brief recap:

• Males are the only ones that can be reliably identified and then only in breeding condition. At this time, they (sometimes) develop breeding tubercles on their gill covers and the front rays of their pectoral fins. These look like a sprinkling of fine salt or sand on the fish. Some people mistake this for Ich the first time they see it. One difference is that the breeding tubercles do not spread all over the body and fins.
• Females are most often identified when your males (quite possibly in breeding condition, but not necessarily with tubercles visible) start chasing them and nudging their sides and vents.

Be aware that males will relentless drive females to exhaustion, so you need to make provisions to remove the female when spawning is done or to provide some shelter in the spawning tank. Also keep in mind that goldfish voraciously eat their own eggs, so you want to remove the spawning material (or the fish) once the action has slowed down.