I've been importing, buying, swapping and breeding Guppies for years. Unfortunately it's very very difficult to get great fish here in Australia although they are available and some reasonable stocks are available if you are prepared to work the line a bit.
One of my key frustrations is that it is near on impossible to buy virgin females. To me this is one of the single biggest "musts" when breeding in particular if you are trying to improve a line. If you don't know who the father is then it's impossible to know the genetic make up of your fish; who knows what you have!
So, I thought I would tap out an article on the method I use to make sure that I have virgin females available for my breeding program.
Female Guppies have a very big give away that to the trained eye is visible at about 2 to 3 weeks old; the gravid spot. The gravid spot is the darker spot just in front of the anal fin.
I typically try to separate my females as early as possible so start the process as soon as I notice a gravid spot and continue the process util I've moved all the females. Some breeds are easier to spot than others.
My fishes drop of fry are moved to a grow out tank from day 2 (I use hang on the outside breeding boxes for the birth). Obviously at this point it is a mix of gender in the grow out tank. At this stage, if I don't have spare, I cycle a single 20L tank in readiness.
The key to separating is that you only move the female fish. The reason for this is that providing you are 100% sure it's a female you move it away from the males. In this way you know that the female tank only holds females ( by default it's possible that the male tank, the original grow out tank, holds both genders). If you were to move the males you run the risk of missing one and leaving it in with the females!
That's it! In point form: