Hi Helene,
I know from breeding appistos, skewed sex ratios was mostly caused by temperatures. Since your tank is unheated it may be caused by tempurature fluctuations, maybe by just adding a heater to stabilize may solve the problem. Of course cooler water is a longer developement time and warmer water is faster. Which makes more males or more females I can’t remember. (I believe warmer makes more males but not sure anymore)
I realize appistos and Paros are a world apart, but both share the exact same sex ratio problems, including soft, acid and usually black water prefered systems.