+1 Amphirion - many myths on this topic
1. Some believe pH - but I forgot/not sure about high pH = more females/males (think its females)
2. First 3 spawns (female) = more males
3. younger male = more males and visa versa
4. Temp - higher temp = more males
I rely on 2 and 4
Sexing same batch:
Bigger fins, longer/slimmer body, more vibrant color (often confusing), bigger head.
Viewed from above: bigger/wider mouth, big and short head, smaller midsection.
smaller fins, shorter and fatter body, often show breeding bars when colored up (not always), smaller head.
Viewed from above: smaller mouth looks more "pointy", smaller head, bigger midsection/belly.
8 weeks is rather too young to sex, though differences are visible. HMs (or any long finned) are usually easier to sex than PK