Female To Male

[Brightestdai]

I have bought 3 different red "females" from various breeders and all have been male. First was 2003, 2007 and 2013. I guess what I'm asking is what do I look for other than stripes? I'm tired of getting full red "female" to male betta's

(Missing part fin is due to a fight with 3 females)

 

[Wildbetta]

Personally from that pic, that betta appears female.  Would need a better pic to be sure though.  Why do you believe it is male?  
 
To answer your question though, you would look at the length of the ventral fins, the anal fin, and the caudal fin.  It also depends on the tail type of the fish (VT, HM, PK ect) as to what to look for.

 

[Brightestdai]

It's extremely aggressive, I've got close to 10 females and none have been this aggressive and they all live peacefully in a 20 gallon. Then when I introduced this little one it jumped out of my hatchery ( I put all bettas in there just for a couple mins to make sure no fighting) and went all betta out of hell on my females

 

[kazc31]

Some females can be just as aggressive as males! So aggression alone isn't really anything to go by!!

 

[Brightestdai]

here's a better pic

 

[Wildbetta]

From that second photo, I would still say it is female.  Females are easily as aggressive if not more at times than males.  Aggression is not a way to determine gender. Solid reds also seem to be more aggressive than other colors for some reason.  There are some females that just can not be kept with other fish. Which is why it is always a good idea when keeping bettas with other fish to have a backup tank available in case the betta does not do well with with the other fish.

 

[Gizaroo2]

Definitely looks female to me. Some females, as mentioned above, are extremely aggressive. I had a crowntail female at one point that would have never been able to stay in a sorority. She was just too mean. 

 

[Brightestdai]

well.... its deff a male, found a huge bubble nest this morning. either way im happy, i have 4 females and 2 males now

 

[Gizaroo2]

Females can make bubble nests too.

 

[attibones]

Does the fish have an ovipositor? That's one way to know for sure... Should be a white sort of dot between the ventral and anal fins.

 

[Wildbetta]

I see an oviposter in the second photo.  There are females that build bubblenests and even ones who release eggs and put them in a nest they themselves built.  I still say it is female unless you can get a full frontal flare pic.