Its odd to me that around here wallmart is the only place that I have consistently seen female bettas.. Even my LFS only has them on ocasion.
I no expert photographer, but I find the best results for me are with no flash and just the light from your tank.Its a little harder sometimes since the shutter stays open longer than it does with the flash, but I prefer the colors you can capture alot better to how washed out and bland they will look with the flash on.
Good luck!