Things You'll Need
Instructions
Look closely at the belly of each fish, whether platy or swordtail.
Check the anal fin, which is located towards the rear of the belly. If you are not sure, watch for the fish to produce some excrement. The fin nearest the exit vent is the anal fin.
Look for the anal fin that is smoothly rounded. The fish with this type of anal fin is a female.
Identify a fish with a different kind of anal fin. If the anal fin comes to a sharp point at the tip, the fish is a male.
Check the gender of fry at an early age if you wish to separate male and female to prevent reproduction. A female can produce up to 80 fry from a single pregnancy. This can quickly overpopulate a small tank if left unchecked. Reproduction in live bearing fish has been reported as early as three months of age.