Looking at fin length alone is not a reliable way to sex bettas, because there are long-tail and short-tail varieties. It's easy to confuse a male plakat for a female; pet stores do this all the time!
If you are trying to sex a baby betta (fry,) then these tips will generally not apply, as sexual dimorphism is not apparent until maturity. (Around 2-3 months of age.) |
The only fin you can reliably look at are the ventral fins.
|