[Peter Finke]

Patrick, excuse me that I did not reply to your questions.
Yes, you are right, contrary to the sex determination in mammmals or most birds in fish or amphibians this decision is not taken in the moment of the fertilization of the egg but in the days and even weeks to come by the influence of environmental factors. We know of the potentially decisive role of temperature, pH and even the quantity of humine substances. I cannot tell you of an exact duration of the period before the decision is taken, but experiments show that it could be rather a long period of several weeks mostly. Quite often breeders complain of whole clutches developing to males or females, and it is most likely that they have put the foundations for that by themselves by a low or high pH, a low or high degree of humine substances or a low or high temperature.
We know of no differences between the Parosphromenus species but there maybe important differences between fish of very different categories.
You can find many additional information in the internet.