Pregnant fish will not be able to produce babies in a crowded tank, and purchased eggs will not hatch. The tanks in Fish Tycoon cannot hold unlimited fish, and when babies are born into a crowded tank, any babies that are born after the limit is reached will be 'eaten' by the other fish. This limit is most noticeable in the Lite version for iPhone, since the tanks each hold 7 fish. In the full version of Fish Tycoon for iPhone, as well as the Windows and Mac versions, you can have a maximum of 21 fish (or 20 fish and the seahorse) in each tank. In the Palm and Pocket PC versions, the tank limit is 20 fish.

To make sure that you will obtain babies from your pregnant fish or purchased fish eggs, put them in a tank that has room for all of the babies to be born. Be sure to take into consideration any magic effects you have discovered that cause your fish to produce more than one baby.