Guppies are live-bearers, meaning they give birth to tiny live guppy fry. The can do so about once a month. These are fun to raise, but you have to separate the fry from the bigger fish or they little ones will end up as the main course for the others in the tank before they'e barely out of the nest.
Newborn guppies are less than a quarter of an inch long, but over a period of about six months the males will grow to an adult size of around 1.5 inches, not counting their tails. Females will typically end up considerably larger, but with the girls most of their size is in the body and not the tail. Overall development depends on genetics, tank conditions and feeding. Starting anywhere from about 1 week to 6 weeks of age, they will begin to display color in their tails and fins. You can also usually tell the sexes apart starting at about 1 week of age.
The quality of the food and the frequency you feed your guppy fry have a lot to do with how they develop. Live foods such as baby brine shrimp, daphnia and microworms will help your tiny guppies grow to their fullest potential. Other good foods for fry include crushed or grated hard-boiled-egg yolk and crumbled flakes of high protein fish food. Feed them a variety of foods four to eight times per day, but feed only as much as they can eat within a few minutes. Overfeeding will result in dirty water and can be harmful to your fish.
Give guppy fry plenty of room to grow. Fish living in crowded tanks tend to develop much more slowly than they would under optimal conditions, and they may never attain a normal size. Dirty water that is high in nitrates is nature’s way of signaling that there are too many fish for the space, and in such conditions the fry’s growth will slow or even stop. Replace about half the water in the fry tank every week to prevent the buildup of nitrates and other toxins and to help to keep the babies growing.
Guppy fry need a reasonable amount of light to develop into healthy adult specimens. The tank should be bright enough that the fish are easy to see, but it doesn’t have to be lit up like it’s noon at the equator. Give them about 12 to 17 hours of light each day to prevent spinal deformities. In addition to light, keep the water temperature around 80 degrees Fahrenheit. This steps up their metabolism and helps them to grow more quickly. When you’re a tiny guppy, size matters, and the sooner they are fully grown, the better their chances for survival are.