platy breeding box

Hi there quite new to this! I took these platies on from a friends who moved away, they are just a portion of the fish she had the others went to a relative. I just wondered if anyone could tell me if they are pregnant or not I tried my best the get photos. I’ve done a bit of research but can’t...

Ok, I have a top fin 3 way breeding box. I bought a platy yesterday who looks as she is about to explode. Her gravid spot is a dark brown, with little black dots. When should I put my platy into the breeding box?