I had one male and two female guppies. A little more than a month ago, the male died because he was being bitten a lot by the females. Maybe there was another reason as well. The female was impregnated by him and still hasn't given birth more than a month later. She looks pregnant, but not ready yet. She also attacks the other female guppy and chases her away from food. They are in a well-planted 5 gallon (19 liters) tank. I recently put the female in a separate tank, so that there is less stress. Will she ever give birth? Why is she acting strangely?
So, although your male has passed, female guppies can actually HOLD sperm for up to 6 months, when they do Hold it for fertilization the typical amount of time held doesn't usually last more than 3 months,but the sperm can last the full 6 mos.The black spot near her rear, known as the gravid spot is typically mistaken as being pregnant but is actually a sign of sexual maturity. This spot will increase in size during pregnancy and as she gets closer to giving birth her body will be more rectangular than oblong (based on picture somewhat shaped, but she doesn't look quite there yet). Gestational period is around 28 days, so think of when he passed awhen he could've last passed sperm, and how the dates match up. Make sure your tank has a ton of foliage ( live or artificial) so she has a safe place to birth away from fish waiting to feast, so her babies are less likely to be eaten, especially in the event she gives birth when you are not around. Guppiez spend most of there time toward the surface so make sure the plants you do have are tall, if not get some, floating plants work great for this. This strange activity could be her trying to set a boundary or territory, preparing for birth to keep other fish away when she finally goes into labor. Good luck, and hope you have beautiful babies! It's so fun to watch them change from looking like a jelly set of eyeballs to an adult.