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Young guppies, also known as "fry", require special feeding separate from the adults, and should be separated for their safety as well.Prepare a 5 gallon (20 L) tank with care, making sure it has everything necessary for the guppies' survival. As with any aquarium, running a fishless cycle is an excellent way to prepare the tank before you introduce any fish. Purchase a breeding net or mesh from a fish store that divides the tank into two sections. The newly hatched guppies will be able to swim through the holes of the net and escape the mother fish, which may try to eat them. If you don't want to purchase a net or are not sure if the net you have has the right sized holes, you may protect the fry by planting several aquarium plants for them to hide in. Females have a dark spot on their abdomen, also called a gravid spot, which becomes much darker once her internal eggs have been fertilized. The eggs will grow inside her for 21–30 days, and her abdomen will become larger and darker as this happens. Transfer the female back to the main tank as soon as you notice she has given birth, so she does not compete with the fry for food or attempt to eat her young. If you are breeding guppies to sell or to produce specific traits, move the male and female you wish to breed into the tank before the female is pregnant. Return the male to the main tank once you notice the darker gravid spot. Note that female guppies can store sperm for months, so a fish you purchased as an adult may be fertilizing her eggs with a male not present in your tank for the first several litters. Guppy fry have tiny mouths and high protein requirements, so they must be fed separately from the adults. Have their special food ready to go before the fry are born, and include both protein sources and vegetable matter. High quality protein can be supplied in the form of baby brine shrimp (not adult), microworms, finely chopped beef heart, and/or a mash of egg yolk and water. Vegetable matter can be used in smaller quantities, and can take the form of infusoria (microscopic organisms that color pond water green), fresh and finely chopped dark green lettuce, or small pieces of boiled spinach. Do not to feed them adult brine shrimp until they have grown into juveniles; if the shrimp remains untouched it is too big for the fry to eat. Growing fry have greater energy needs than adult guppies as well as smaller stomachs, so feed them tiny amounts of food about 6 times daily. If the water grows dirty or cloudy despite frequent water changes, reduce the amount of food and make sure it is small enough for them to eat. Once your fry have been born, you may wish to change 10% or less of the water every couple days, instead of doing a larger and more stressful water change every week or two. Young fish are especially vulnerable to water changes, and the high frequency of feeding can cause waste to build up in the tank quickly. After about 2 months, the fry should be considerably larger and will only need to be fed 3–4 times a day. After about 4–5 months, they can be switched to an adult diet and fed 1–2 times a day. You may choose to alter the diet slowly to increase the odds of the fry accepting flakes, gradually replacing other parts of the diet with flake food. Different varieties and individuals will grow at different rates, however, and you should not move the fry into a tank with adult fish until they are close to the same size. You may be able to switch to larger food such as adult brine shrimp earlier if your guppies are growing at a good rate. Keep feeding the juveniles high quality food, however, and watch them to make sure they eat the new food.
Prepare a separate breeding tank. Divide your breeding tank into two sections using a breeding net. Move pregnant females to the breeding tank until they give birth. Prepare special food for your fry. Feed the fry several times a day. Change small amounts of water frequently. Reduce the frequency of their feedings gradually.