One always should bear in mind that the sex of a fish-individual is not determined in the moment of fertilization (as we know it with mammals or birds) but happens days or weeks later by environmental factors. Temperature, pH, the content of humic substances and other factors: all could contribute to the decision. We sometimes have an outcome totally of either one sex or the other; then one or several of these factors have been near to a limiting value during the first life-time of the young.
Another thing is the fact that in the trade of wild caught Paros sometimes only one sex is sold. Some years ago a German wholesaler asked me by two photos which species it is; the official name was – as usual – “deissneri”. It was spec. Sentang (of course), but the striking fact was: the photos showed only male individuals, about 200 or more (see photo of similar situation below). I had for long no explanation for this phenomenon. Nobody has the time or reason to select the fish artificially. But there is a worrying explanation: More and more the habitats are damaged by many factors (e.g. chemical pollution). Often we find Paros today no longer in their traditional natural rainforest habitats (because they are destroyed) but in poor side canals of roads and other water bodies heavily changed by human activity. Take light only: If the rainforest is gone, there is much more light than before. The huge mass of leaves and wood formerly falling down into the waters is missing. The pH changes, often the temperature is higher (or lower) than before. Therefore we cannot exclude that such monosex offspring is to be found in “nature” already.