There has been considerable discussion in aquatic circles about the factors that may or may not influence gender determination in developing fry.
Now some evidence from the direct experience of at least one killikeeper has tested the link between water conditions and gender in killifish.
In September 1996, Gary Elson, writing on the electronic Killifish Mailing List, wrote that he was spawning a young trio of Aphyosemion zygaima. In the spring, water conditions were pH 7.0, hardness approximately 90 ppm and 18-19C. With fry mortality of near zero, he got an even sex ratio.
During the summer, with the same fish, spawnings were larger, although egg production “was awful” above 23C. With water parameters of pH 7.4, hardness 140 ppm, and temperature 22-24C, he got almost 100% males. Gary also noted that he had a high mortality rate, so that it was possible that the larger males were killing the smaller females.
In February, Gary posted again on this subject, with the following: “Back in September, I posted a question about skewed Aphyosemion zygaima sex ratios (15-1 males). I was hatching and rearing my fish at pH 7.4, 120-140 ppm. Following the excellent debate, which led to a lot of fascinating information about post hatch sex determination, I started raising some batches in pH 6.5, 30 ppm filtered rainwater mixed with tapwater.
“The results are in! 9 to 1 females!”
— GCKA Newsletter, April 1997