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Remove fry that show signs of deformity and illness. Avoid medicating your tank, as you risk harming the fry. Place your baby guppies into your regular aquarium at 6-8 weeks old. Separate males and females if you don't want more babies.
Baby guppies are especially vulnerable to illnesses caused by bacteria or aggression from other fish. Don’t leave sick fish in your tank, as others may become ill, as well. You can either attempt to nurse the sick fry in a separate tank or dispose of them, as they're unlikely to thrive. If you notice the following symptoms, remove the affected fry from the tank:  Bent or warped spine Sores Tail rot Fin rot You can find medications to help treat common illnesses in guppies. However, these medications aren’t appropriate for fry, unless you are very knowledgeable about dosing fish. It’s better to remove ill fish from the tank rather than risking the health of the other fish. You can try to treat the sick fish in a separate tank. However, they’re unlikely to thrive if they’ve become ill at such a vulnerable stage. Most baby guppies will be too big to eat at this point. Make sure they’re bigger than the mouth of your adult fish before you put them in the aquarium. Otherwise, the bigger fish may still eat them. Your fish will continue to grow until they reach 6 months old, which is considered adulthood. You may notice some growth after your fish is an adult, but it will be minimal. Female guppies usually start to reproduce at 2 to 3 months of age. Additionally, they can store sperm for up to 3 months. If you don't want more babies, you'll need to separate your male and female fry once they reach about 6 to 8 weeks old. You'll be able to determine the sex of your baby guppies when they're about 1 month old. Male guppies will have a long, rod-like fin on their underbelly, which is called a gonopodium. Females will have a dark spot called a gravid spot on their underbelly.