Many of the small tetras are similaly marked, but a deeper body and noticeably wider when viewed from above is a good sign that you are looking at a female. However, in reasonable conditions, a lot of these fish are free spawners, so the females never really fill with eggs and can look just as slim as males.
The circling with erect fins and "flashing" are typical courtship/breeding behaviour patterns, particulaly if one of the fish is doing the majority of the circling, (male). Be aware also that 2 males will on occasion, display to each other to try to raise their staus in the pecking order, in this case, both the fish tend to be very actively displaying.
Hopeing that you can answer my question folks, i have 6 serpaes in my 15g tank and they are all very well much at home there. I've had them about 5 monthes now but i have no idea what's so ever on there sex. I understand that males and females have all the same markings the only difference being that the females have a slightly plumper body is this correct ?. If that is true i have only 1 female possibly 2, out the 6 the only thing that i have noticed is that asll of them like flash to each other. The dorsal fins are really erect and they seem to go slightly to one side when they go around each other, there bodys sometimes touching. What i really wont to know is that behavior for 2 males or is it the courtship of a females, they are doing it all to each other just lost got no idea to who is fancying who 
. So any information would be gratefully recieved Shanna
/www.aquarticles.com/articles/breedi...ck%20Tetra.html
many thanks wilder
. They are chasing anything that goes near there little hiding place so will keep you posted on the romance 
