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For krib types, it's relatively easy, even at a small size, to sex them. The females' pelvic fins are rounded or squared off, much more blunt than the males. The females tend to have much more belly color or irridescence (depending on the species). In moliwe, the females have pastel-colored bellies. The females also have shorter, chunker bodies. The males are longer and sleeker. As males age, their dorsal and anal fins are pointed, the females are more rounded.
They are much like other kribs. Females have a more rounded belly and the males are larger and more bullet shaped. In some species, the females develop dorsal spots early. Males rarely have them on the dorsal.
I can easily identify that one of them is female - she has a round belly and its rosy-purple. The other guy gives me some doubts - he is more bullet shaped but he has a yellow stripe and his belly is kind of pinkish blue as well, does this mean that this can be a female as well?
Everyone here jumped on the band wagon to help you out before I had the chance! And, I believe, it's all very good information.
If you're still having doubts about the gender of the one fish, Wendy (Finatic Las Vegas) is right on. Look at the fish's pelvic fins. If the first ray is the longest: male. If the second or third ray is the longest: female. It's really that simple.
Sometimes it is really quite difficult to tell these guys apart at a young age. Unless you can really get a clear look at the ventral fins, they don't really seem paddle-like until the fish is more mature. Again, I think that it is a matter of trying to use several diagnostic criteria to get a more certain perspective. Belly color sometimes works in young fish. Caudal fin patterning may start showing up early (As Ted mentioned, females often start showing those black spots in the caudal early). But the ventral fin is probably what to really key on!
Neil