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Video: Guppy: Breeding

I have been keeping guppies in a common aquarium for 6 months, but they have never given offspring
I think the answer lies in the youth's chances of survival, not in the absence of offspring from your guppies. You should see that the males constantly show increased attention to the females, after which the females get fat. If, after about thirty days, the females' figures become graceful again, then the offspring have appeared and should be clearly visible in the aquarium. If you don't find them, then the other fish in your communal tank are regularly receiving fresh food from your guppies.

Male guppy
To be sure to achieve the "result" start a small, separate, well-planted aquarium and, as soon as you notice that your guppies are pregnant, plant them in this peaceful and quiet place during labor. The fry will feel safe in the midst of the plants, and you will finally have the desired offspring.
Using a separate aquarium as a nursery is more merciful to a pregnant female than placing her in a small plastic cage for this time. Also, never touch a pregnant fish towards the end of pregnancy.

If your guppies are different species, they can interbreed and give birth to mestizos. Therefore, if you want to get fish of a certain color or with a certain shape of fins, you must separate them from the rest.