First, Find out the age of the fish. If a pleco is not yet fully developed, then it will be difficult to sex the pleco. Make sure that the pleco in question is a mature adult -- usually at least four to five years of age.
Next, Examine the pleco's forehead. Males have flat, level foreheads. Females tend to have rounded foreheads that bulge out slightly. When a female is ready to spawn, her forehead will appear plump.
Look at the fish from above. Just as the male's foreheads are more level and linear, so are their bodies. They do not appear "fat" or "plump" in any way. Females, on the other hand, have more curves and tend to be a bit larger than the males when they are full grown.
Lastly, Check for barbells. These are spines that grow at the sides of the mouth. Both sexes develop them in some species of plecos, but in others, only the males develop them. If you have a pleco with no barbells, then it is a female. Also, if the barbells appear stunted and smooth, then it is likely a female.
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