It's not really a fertility issue, but rather a space issue. In the Windows and Mac versions you can have a maximum of 21 fish (or 20 fish and the seahorse) in each tank. If babies are born into a crowded tank, any babies that are born after the limit is reached will be 'eaten' by the other fishies! So, if you have 15 fish and the seahorse in your tank and make all 15 fish pregnant, 5 babies will be born and survive, and 10 babies will be 'eaten' by the other fish.

This issue must be carefully considered in your breeding choices, especially if you have made purchases or discoveries that allow fishies to produce more than one baby with each pregnancy.