QUESTION:

I have Betta fry that are now almost 3 months old and was wondering at what size it is possible to start sexing them?

ANSWER:

There is no fixed size at which you can start judging. A lot depends on your experience. For myself, I am able to judge with a 90% confidence at 3/4 inch at the rate of 12 samples per minute looking at a group tank, but then I have gone through hundreds of thousands (literally) of betta fries. What I can do is to share with you the way I spot the fishes, you will have to try them out and see what size you are confident with......

• Rule 1, for long tail show varieties, a long tail FULL-GROWN specimen is a male. Others are females. Exception to rule, short finned varieties, and there exists very occasionally, long finned females, but these females are easily differentiated by their lack of volume of fins.
  
  
• For light-bodied fishes, such as the traditional cambodian, by virtue of the slight translucent bodies, it is possible to identify females as those that show the egg sacs. Females start to develop egg sacs at 3/4 inch, and healthy specimens should definitely show egg sacs at 1 inch size. The egg sac will appear as a yellowish triangle located behind the round stomach (belly) and under the main spine of the fish. Exceptions to using this rule, malnourished females may not grow egg sacs. There are also some tendencies for some show lines to be very weak and for delayed egg formation. I personally will use this method as it is by far the most accurate, if it is possible of course.
  
  
• This 3rd method is an derivation from method 2. Since females develop egg sac, their body will be stouter. This is visible when viewing the fishes from the top and from the side. Besides the considerations listed in (2), we must also take into account fishes with shortened bodies, either because it is the dt (double-tail) variety, or because of genetic defects (such as the so called "bonsai" bettas). For dt fishes, the stouter bodies can be differentiated by viewing from the TOP but less so from the side. I use this method for darkbodied (that is the non-light bodied) fishes.

Some other methods used by the others, but which I found to be have quite error prone:

• a fish with an egg spot, a small white spot located below the belly, between the 2 ventral fins will be a female
• males have longer and fuller ventral fins
• the "slope" of the male's head is steeper
• head of females are less "black" for iridescent fishes (NB: The best coloured females should not be any different from the males where colours are concerned.)
• males tend to have their gills sticking out from their gill covers (this method gets more accurate as the fish ages, but by then other signs will be more obvious)