To mate, there are a few conditions that you'll need to meet.

Of course, waiting for your fish to get to adulthood may take some time. Once an adult, make sure they're well fed, the tank is clean and make sure you have half the coins it usually costs to buy an egg from that fish (example: Auriga eggs normally cost 14 coins, so mating costs 7 coins). If you miss any of those conditions, you'll get the "Almost There!" screen until you figure out what condition's not being met. As you can see below, we had to wait for our male Auriga, Rascal, to mature enough to mate. Once ready, use the pointer to select either fish and click the Mate button.
Given that you have an 80% chance that the mating will produce a baby fish, you may not get an egg after the love ritual. 
If it works, you'll get a "New Baby Fish Hatching Soon!" announcement that you'll be able to post to your Facebook page. The one bad thing is that eggs take half the time it takes for a new fish to become an adult. So with the Auriga time to adulthood being eight hours, a baby egg takes four hours to hatch.

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