I have 5 different spawns from the WC pair. The spawning conditions vary from spawn to spawn as I am trying to determine the optimal sex ratio as well as try to manage spawn size.
From the 5 spawns, the pH ranges from 4.7 to 7.0 (the spawn at 7.0 didnt even hatch, nonetheless they did spawn at that pH). Temperature ranges from 76 to 80F. Conductivity ranges from 65µS/cm to 250µS/cm. The 250µS/cm was the spawn with pH of 7.
Here is what I have concluded so far.
1) Low conductivity gives larger spawn (more eggs hatchout). At 65µS/cm, the spawn size was 300+, at 180µS/cm the spawn size was only 100 even though the amount of eggs observed was much higher (too many to count). The cut off is probably around 300µS/cm even with favorable pH.
2) pH and temperature play hand in hand in determining the sex ratio. At high temperature, the sex ratio skews toward male. With pH of 5.5 and temperature of 80F, a 75+% male favored ratio were observed. Currently I am waiting to count the July 31st spawn of 100+ fry that were kept at 76F with pH of 5 for 6 weeks. Just a casual scan of the tank tells me I am on the right track for more females but still short of the ideal 50/50. The Oct. spawn was then set at pH of 4.7 and Temperature of 76F with the conductivity around 117µS/cm. The spawn size is around 200 (since the wholesaler indicated there is a renewed demand for this species so I adjusted the conductivity accordingly). My plan is to maintain the pH at around 4 to 4.5 for 6 weeks at 76F. Sexual characteristic of the female (silver spots around the genital papilla) starts to appear around 7 weeks. By the 12th week, the entire group is easily sexable.
One thing I would like to mention is I read somewhere that someone was able to get 90% female. The only way I can see that happening is at temperature of 70F and pH below 4.