I found that the young male fancy guppies will always want to breed with the females, even if they´re pregnant. You should keep one male with at least two females.
I have been keeping Guppies nigh-on ten years now. The feeder Guppies, which are pretty close to the wild variety, are the hardiest and most affordable. They also breed like crazy! With Fancy Guppies, I find that the young males are preferable for breeding. The older they get, the less inclined they are to breed. Here's something I have never seen before; I have a male Guppy named Liam who dedicates himself to only one female, usually a dark-colored one, and pesters her to breed incessantly. He ignores all other females in the tank!
The Fancy Guppy does not like to be over crowded but likes to be kept in schools of its own kind.If you just want to keep Fancy Guppies, get an 80 liter tank and pick 10 males and about 2 to 5 females. This way the males won't be as interested in chasing each other and tearing each other's fins. But do not get just 1 female, it will stress her out being with all the males, they will not leave her alone and in this way she may die.
Guppies are beautiful, small, tropical fish, but there are two downsides: First, they will overpopulate your aquarium in no time. Second, they aren't as hardy as they used to be, and die very quickly if you don't take good care of them (but who would not take good care of their fish).
I have found that Guppys are NOT easy to keep. Whenever I try to breed them the female always dies and I end up with a couple males and no females. None of the males die. I have 10 male Guppys.
This is my favorite fish. They are very hardy, strong, eat everything, easy to breed, colorful, peaceful. They are good starting fish. I have very colorful guppies bred from a beautiful japanese tropical blue male and a beautiful snakeskin female. When they dance around the female you can see the colors the best. Sometimes the female wants to hide because if the male chases her too much she gets tired, so they need plants. The little fish that are born can hide in Java Moss (Vesicularia dubyana)