Well, first of all... female guppies are already born with a gravid spot to start with. And it's clear of less clear to see at each individual female. But after 2-4 days a slight spot should be seen. And remind you that a gravid spot doesn't have to be black.
I'm sexing guppies and other livebearers (with the exception of goodeids) for many years at an early stage like that. With xiphophorus strains it can be a bit tricky because a sex change from female to male can happen.
Young females and males tend to grow up with the same speed. But males can develop their gonopodium between the age of 4-8 weeks. This differs between the individual males. Once a juvenile male has developed a gonopodium, it will be functional. So, in a tank with mixed genders, juvenile females can get pregnant at an early stage. Which results maiinly in small remaining females. I'm breeding for shows and competitions as well and you just can't offer smaller females to be judged. At a competiton or championship when you offer pairs, the females should always be larger than a male. So, letting females grow up seperate from the males ensures you to have fullsize females. It also ensures virgins which have a better potential to carry during pregnancy.
Devoted aquarist ever since early 1970's...
Specialized in livebearers &
specific breeder of rare livebearers...