| | Re: Guppy Breeding Tanks
I wouldn't blame your tank or your fishkeeping skills until I knew the history of these Endlers. Sex ratios can be altered by pH, temperature, and inbreeding. I'd put my money on inbreeding, since it is pervasive in the aquarium hobby. Everyone just breeds siblings as if there were no consequences.
I'm still trying to find this scientific reference in my files referring to Zebrafish sex ratios: "WIK lab strain went from 80% females to 72% in males after inbreeding to F=0.375 (Brown 2012)."