Look closely at the belly of each fish, whether platy or swordtail.
"Red wag" is a term relating to two kinds of community aquarium fish. The swordtail and the platy both produce red fish with solid black tails. Both of these types of fish are referred to as a red wag variety. The swordtail and the platy both make excellent members of a fish community, and are live bearers, meaning the female gives birth to live fry. Whether you are observing swordtail or platy fish, you can determine gender of the fry at an early age.
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.
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