I should remind you of the fact that the decision on the sex is in some aninmals (including fish) not determined by the genetics in the moment of fertilization, but much later by environmental factors. I wrote about this several times already, including this website. In Germany, the biologist Steinberg has investigated and published on this issue especially in the case of fish. This is an issue of high importance to the fish industry for in many cases of species it is of great importance for breeders of fish we need as our food to produce more females (eggs!)than males (milk!). There is no doubt in general; the difficulty lies in finding out which factor is responsible in an individual case situation.
The most important factors that have been found to be relevant for this in fish are the pH, the share of humic substances, the temperature and the general composition of minerals. This explains spawns with dominating or pure males resp. females. Every group of fish has its own optimal balance of these factors. Differences among the Parosphromenus species exist, too, but they are slight only.
As a matter of practical help every breeder is advised not to start a specific breeding with extreme values that are conveniant for him or her (e.g. lowest or highest possible pH resp. temperature, no humic substances or abundance of them, or an unusual composition of the mineral contents, for instance by using a water with too high a share of Ca) but to prepare the set up well informed about the ecological situation at the original locations. Generally speaking, too many spawns are performed in water with too high a mineral share, too high a pH, too low a share of humic substances and to high a temperature. Mind, that the pH of the tropic waters in blackwater regions are never without a high share of humic substances, are often of lower temperature in the rainy seasons of mating and rearing the young, when the mineral content of the water is most similar to pure H2O.