In re: to sexing these -
There is no sexual dimorphism externally, at least not known. But some in species of Hoplias, when the male grows quite large and is mature, the anal fin may fold sideways a little bit and form some sort of a scoop...some of the Hoplias species have sex chromosomes, so they could be sexed by a karyotypic test. To date noone has ever looked into the aimara karyotype so there is no information wether they have a sex chromosome. Also, in the case of fishes, most people would have to kill the specimen in order to make the karyotype test. Some researchers would be able to do it only with a blood or fin-skin sample, but these are only experimental fields in Biology now. The only way I know of is to dissect them and look directly into their gonads.