Guppy moms produce new batches of young on a regular basis, but the babies can disappear from your tank mighty quick, since the bigger fish think they’re tasty. Keep them separated and feed them right, though, and you’ll have a nice bunch of colorful youngsters in a few months.
A female guppy will give birth to live babies about once a month, give or take a little. The tiny fish, known as fry, are no more than a quarter of an inch in length, head to tail, but at about 6 months the females end up around 2.5 inches long and the males about an inch less, not counting their tails. You can begin to see some color in them when they’re anywhere from a week to a month-and-a-half old, and you can normally tell what sex they are by the end of the first week or two.
Proper nutrition is essential for raising guppy babies, though they can live on commercial food such as store-bought dried flakes. The problem is that such a diet holds back their development, and they’ll be slow to grow and get any color. Add a variety of food to their diet and your fry will develop much more quickly. When they’re tiny, give them the finely crumbled yolk of a hard-boiled egg and tiny bits of pulverized beef heart. As they grow, add microworms, daphnia and baby brine shrimp. They also enjoy the crushed insides of cooked green peas.
Too many fish in one tank will slow the growth of your guppy babies. Fish waste produces nitrates, a natural signal that too many fish share the space. The result is that the little fish stop growing, or slow way down, so that they are making fewer demands on the environment. Keep the fry in their own tank, one that's at least 5 gallons, and change out half the water every week. As the fish grow, change the water more often and move them into a bigger tank when they’re about an inch or so long.
Water temperature has a major impact on the babies. Keep them on the cool side, around 74, and they’ll grow, but slowly. Step it up to about 80 and their metabolism takes off, causing them to eat more and grow faster. Your guppy babies also need light at least 12 hours daily, along with some dark time so they can rest. If they don’t get enough light, they’ll end up with deformed spines. You don’t have to use a spotlight on them, but the light needs to be bright enough that you can easily see everywhere in the tank.