When Can Baby Guppies Go in the Main Tank?

Baby guppies, or fry, will be able to leave the nursery tank and go in the main tank when they are big enough to not get eaten by the adult guppies. This will depend on the size of their other tank mates, but 1/2” long is usually a good rule of thumb. 

When your guppies give birth, the new fry will have the best chance of survival if allowed to mature apart from the adult fish. The question then is when to reintroduce them. I will go into several important factors to consider as well as the best way to do it.

When Can Baby Guppies Be Put Back In The Main Tank?

If you find that taking care of two separate tanks is a nuisance you will want to put your baby guppies back in the main tank as soon as possible. At the same time, you don’t want them to get eaten because you put them in too early.

The three factors that will decide if your fry are ready to return are:

  • Size
  • Age
  • Health

No matter what, it will be at least several weeks before your baby guppies are ready to go in the main tank. In the meantime, just be sure to feed them regularly and monitor them for the below aspects.

Green Black Orange Clear Guppy


The biggest determining factor is the size of the young guppies. When they are first born, the adults will eat them either on accident or for some survival of the fittest reason. Guppies, both adult and child, will eat whatever they can fit in their mouth.

Now you see the first factor. When the fry are bigger than an adult guppy’s mouth they can go in the main tank. The fry should also be bigger than the mouth of any other fish you have in the main tank.

If you have a hard time estimating the size of the adult fish mouths here is a rough rule of thumb. The baby guppies should be at least one inch long from head to tail.

How long this takes will vary, but is affected by how often you feed them, the water quality of the nursery, and water temperature. If you feed them well and keep the water temperature on the higher end of their comfortable range you will speed up their metabolism.

Feeding schedule aside, the age of the guppies is a good indicator as well.


On average, once the fry are a month old they will be mature enough to join the adults. Since the size of the guppies is the more important factor and it is based on age, this is mostly a baseline.

No matter how fast or slow they grow, by one month old they should be big enough to not be mistaken for food by the adult guppies. If you think they are big enough before one month you can move them to the main tank.

The earliest you could move them is two weeks old. At this point, they will have grown some. The adult guppies may recognize them as not prey but it is riskier.


Even when size and age are at acceptable levels you must consider the health of the guppies. If your new guppies are infected with a parasite or bacteria you don’t want to spread it to your main tank.

Your fry can develop all manner of health problems due to stress, water quality, water temperature, and other factors. Do your best to keep your fry healthy in their nursery tank.

Be sure to keep an eye on your guppies as they grow. Pay special attention if any begin to act out of the ordinary. Look out for irregular spots and colors. Other symptoms to watch for include:

  • Lethargy
  • Scratching
  • Peeling skin
  • Bloating

This is another reason to give your new fry at least a month. In that month you can watch for any symptoms and deal with them as needed.

What Is The Best Way To Add Fry To The Main Tank?

Once your fry have met all three conditions it is time to add them to the main tank. However, you don’t want to just throw them in there. Here are a few things you can do to ease the transition for your baby guppies.

  • Gradually match water conditions
  • Include lots of hiding spaces
  • Feed the adults

Since stress is one of the potential causes of health problems you don’t want to stress out your little fry. Following these methods will ensure your fry have the best chance to acclimate to the new tank.

Gradually Match Water Conditions

The water conditions in your main tank are likely similar to the ones in your nursery tank. In case they are not, you will want to gradually match the two water conditions.

There are two ways to do this. The first is to mimic the main tank’s water conditions in the nursery tank. The second is to place your fry in a plastic bag of their own water then transfer the bag to the main tank.

The bag method will equalize temperatures after a bit. Then you will want to mix in some water from the main tank. Once they’ve had some time to acclimate you can take them out of the bag and place them in the main tank.

Include Lots Of Hiding Spaces

Another helpful thing to do is to provide a lot of hiding spaces. Your fry will be less stressed if they have spaces they can get away from the adult guppies.

Floating plants are one great way to do this. However, any area that the adults can’t get to will work as well.

Feed The Adults

Lastly, you should feed the adult fish before introducing the fry. If you just add the fry, the adults may confuse it as feeding time.

By feeding them beforehand you should reduce any interest in the new arrivals. Be sure not to feed them too close to when you add the fry.