sexing cardinals

[dan223]

Im wondering how hard it is to sex them an at what age or size you can tell.

 

[jd474]

it's not 100% conclusive, but mature males (1 year or so) tend to have a squared off jaw (instead of V-shaped). but the best way is to find an older male and place new fish with him. either he'll start to fight or mate, immediately. juveniles are difficult to sex, but you can always buy groups of 5 or 6 and wait for nature to take it's course, as they will pair up.

 

[danch]

The fighting bit depends on species of cardinal. More massive jaw and 'jowl' is subtle, but can give it away. Only sure thing for all species is mouthbrooding.

 

[candeewolf]

I've heard this (and I notice it is the case with my banggai cardinals), that the male typically has a longer top (dorsal?) fin that goes towards its back tail. I haven't really noticed much of a difference in the jaw between the two except when he's got a ton of eggs in his mouth. AFAIK, 2 males will constantly fight, 2 females will hang out together and beat up others, and male + female will mate (mine did almost instantly. By that token, jd474 is probably right: the most accurate and for sure way to do it is to just let them pair up, and remove the others. It's typically why mated cardinal pairs are usually in high demand.