Reproduction
Guppies bear free-swimming young after internal fertilization. Gestation is often approximately four weeks but varies. Female sperm storage means a mature purchased female cannot be assumed virgin.
Controlled line setup
Use clearly labeled independent breeding tanks; log mother, father, line ID, birth date, generations, suspected phenotype segregation, grow-out conditions, photos and losses. Separate sexes early, revisiting sex identification during growth.
Health versus selection
Select for fertility, behavior, vigor and intact swimming ability as well as color and fins. Carefully planned outcrosses can protect diversity. Controlled research has found higher Gyrodactylus infection burden in strongly inbred guppies.