I have had a female guppy now for 3 weeks. She has had a gravid spot the whole time, however, it disappears after the tank has been in the dark for a few hours. The spot then comes back when the lights have been on for a while. There are also 2 male guppies in the tank but pay no attention to her whatsoever. How come she isn't getting pregnant? Plenty of plants in the tank, water changed a week, fed twice a week and all water parameters fine. What's going wrong?