how to tell if neon tetras are male or female?

Answer 1

It's easiest to tell when you have a group of them together and they are of a pretty fair size. The females are generally the larger fish in the group and are also plumper than the males. Females get quite plump when they are in breeding condition and full of eggs. In this photo of Cardinal tetras, a close relative of neons, the top fish is a female and the bottom one a male.

http://www-personal.umich.edu/~wfink/axel.jpg

MM