I have what I call a retirement tank for my female guppies,
When most female guppies reach their point of no longer being able to have young they will still look hugh like they are pregnant all the time, But you will never get fry from them,,,
They are all fried out (so to speak),,.. So picking hugh pregnant looking females at pets stores are not always a good thing, The smaller females are your best ones to purchase,
Our walmart here is bad for selling old females, people buy them thinking their gonna have babies and never do or die shortly thereafter leaving most think she died giving birth ar the lack of giving birth.
I separate them at this point and let them live out the remaining time without any harassment from the males,
Guppies only live approx 2 yrs, The higher their temps the faster they live and quicker they die.
(Live fast and die young)