I recently picked up 4 wild-caught rivulus, that were sold as "Rivulus speciosus", which I suppose are now Anablepsoides, or whatever. They are quite small, only an inch or so, though I read that this is close to the max size of this species. I was looking at them closely, and I can't figure how many males/females I have. They're all a light tan color with small red flecks. Though, 2 of them have a very faint turquoise sheen on their unpaired fins. I'm assuming these are males, but I'm not sure if the others are females, or just submissive males. Given that most Rivulus aren't that colorful to begin with, is there any definitive way to tell the males from females? Thanks.

Did you look to see if there is an ocellatus on the upper caudal peduncle? That usually designates the female.

Yeah, I just emailed Frans & he showed me some pics of a female, and I most definitely have 4 males. It's particularly annoying, as these actually are a nice looking little fish, especially for a Rivulus. I spoke with the supplier & he's trying to find some more.