Sexing bleeding heart tetras

[UK rams]

Hello everyone. Could anyone tell me at what size do bleeding hearts show sexual dimorphism? I am wanting to buy some to breed but they aren't fully grown and all look female as none have the long sickle shaped fin of the male. I would say they are about half adult size.

I don't mind buying a group of youngsters if there's a chance they are males and females but we all know some wholesalers like to sell just one sex of certain fish so we can't breed them. I don't want to buy a group of one sex

[Fish Folk]

From what I can gather, this species is rarely bred in aquariums due to its slow egg release strategy. Instead of dropping many hundreds of eggs at once with the reality that many fry will be eaten before reaching maturity, these particular Tetras drop eggs slowly over long, extended periods of time. This leads to fry growing at different rates in relatively small numbers. If you spawn them, a full spawning report would be very helpful to the hobby.

I would personally just buy a mature group with clear males and females for a breeding project. As a rather general rule, most schooling fish mature to breeding age either within 6 months, or within 1 year. This accords with seasonal breeding windows in spring and summer. 

You can read an interesting taxonomical study under "Notes" at the bottom of this webpage. It is most interesting to observe that the "Bleeding Heart Tetra" descriptor encompasses three separate species.

Occasionally links from this site will provide detailed information; however, I am not finding exactly what you're looking for.

Helpful notes on breeding challenges here, along with corroboration of information from these other links.