>I would appreciate a response.
In most species it is rather easy in nearly adult fish as they tend to
have longer dorsals and anal fin in the males. However, some
species have just larger fins in the males (like Melanotaenia
irianjaya), others have only slightly larger fins in males (like
M. lacustris). Therefore it is necessary to have both sexes to
find the differences in these species.
It is quite more difficult if the species aren't adults. But if
you compare them with each other (of course you need to have
or see a couple of them therefore) you may also find the fin
differences.
Harro