Guppy Fry Not Growing: 7 Must-Know Solutions

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I remember when I first noticed that my guppy fry weren’t growing. I had three of them in the same tank as some adult females and males (the children were from a previous batch, so they were all different ages). And for the longest time, they barely grew at all. Luckily, as time passed, I learned why this was happening.

Guppy fry typically stop growing due to space issues in the tank. If the tank is overcrowded, the guppy fry won’t be able to compete over food. On the other hand, too spacious tanks will pose an issue of finding food. However, in some cases, it is merely genetics or inappropriate water conditions.

As we proceed, I will teach you seven steps you can take if your guppy fry stop growing. That includes testing the water pH, ammonia, nitrates, and nitrites, using the well-known API Aquarium Test Kit (link to Amazon).

Also Read: Guppy Fry Care

Why is my Guppy Fry Not Growing?

Guppies are beautiful, especially when they grow into adult fish. They are not that impressive in terms of appearance, and there is little point in keeping them around if they have refused to grow. 

Though, that isn’t an excuse to throw away all the slow-growing guppy fry in your tank. If you can identify the factors preventing them from growing, you can take steps to enhance their growth. Some of those factors include:

1. Space Issues

Some people think that young fish will grow to fit the size of the tank. In other words, if the tank is big, the fish will grow to their full size. But if the tank is small, they will limit their own growth to fit within the available space.

But that isn’t true. Fish cannot deliberately enhance or limit their growth. That being said, the size of an aquarium can have an indirect impact on their maturation. You have two elements to consider in this situation:[1]

  • Too Little Space – A small tank is a source of stress for fish. It doesn’t allow them to swim as freely as they would like. Small tanks also encourage toxins to spike. This can either reduce the rate of growth or stunt a fry’s growth completely.
  • Too Much Space – Most aquarists do not expect large tanks to pose any threat to young fish, but they do. When a tank is too large, the fry have a much harder time finding food. They also expend a lot of energy in the process, which can limit their growth.

2. Environmental Factors

The conditions in an aquarium will directly affect the health of your fry, determining whether or not they reach adulthood. 

Fry that survive to adulthood despite the poor conditions will suffer the consequences, including poor health, deformities, greater vulnerability to diseases, and slow or stunted growth. 

Some notable elements that can stunt a guppy fry’s growth include:

  • Water Changes – Whenever an aquarist fails to make regular water changes, they permit the quality of the water in the aquarium to deteriorate. This exerts stress on the fry, making them sick and limiting their growth.
  • Food – Starvation can stunt the growth of guppy offspring. The same thing can happen to fry that are forced to eat poor quality fish food. You have to feed them on time and in quantities that can satisfy their hefty appetites. Otherwise, their health will suffer.
  • Parameters – Young guppies are even more sensitive to the wrong parameters than adult guppies. If the temperature, pH, and hardness are wrong, the fry will either die or suffer consequences associated with their health, including slow or stunted growth.
  • Toxins – Toxins like nitrites, ammonia, copper, and chlorine are a death sentence for fry. But if your fry survive, slow or stunted growth is just one consequence among many that will manifest in your young guppies.

Also Read: Why Are My Guppy Fry Dying?

3. Genetics

Some guppies have genetic anomalies that won’t permit them to grow. You cannot resolve a genetic issue in guppy fry. In most cases, it is just a fluke, an element of bad luck. Fortunately, adult guppies breed quite easily. So you can just try again.

4. Illusion

Are you sure your guppy fry have refused to grow? As was noted above, guppies breed pretty easily. It is possible that the adult guppies keep introducing new offspring every few weeks, and you are actually looking at new fry.

The best way to approach that scenario is by counting the guppy fry in your tank. If you see that the number keeps going up, then it should raise your suspicion that you’re looking at different guppy fry each day.

What to do if Your Guppy Fry isn’t Growing Properly?

It doesn’t take nearly as much effort as you think to help a guppy fry that isn’t growing properly. In most cases, all these creatures need is a conducive and stress-free environment with the appropriate conditions. Try to keep the following in mind:

1. Setting the Right Temperature

Of all the parameters, the temperature is one of the most important. Guppies can live in tanks with a temperature ranging from 70 to 80 degrees F. However, the ideal temperature is 80.[2] The fry can survive in cooler tanks, but they will grow slowly. At 80 degrees F, the fry have a much higher metabolism. This allows them to grow at a faster rate.

It is also essential that the temperature remains stable. Frequent fluctuations may stress your fish and stunt the fry’s growth. That is why I use the Cobalt Aquatics Neo-Therm Pro Aquarium Heater (link to Amazon).

That device allows me to set the temperature to 80 degrees F, and it will be maintained in that range. The heater is a flat surface heater that can sit on the bottom of the aquarium without tipping over. It is also compact, easy to use, and durable.

2. Picking the Proper Diet

Your fry can only grow into adult fish if their diet meets their nutritional needs. This means that you have to feed them with regularity and in a way that suits their appetites.

Guppies are omnivores. They need a balanced diet that includes animal and plant matter. You can feed them blood worms, baby brine shrimp, daphnia, and flakes, to mention but a few.[3] Unlike adult guppies, guppy fry can eat as many as eight times a day. They have large appetites. 

They cannot grow unless you satisfy them. Naturally, because the fish are so small, you have to grind their food before sprinkling it all over the tank during mealtimes. If the food is too big, they cannot eat it.

Some aquarists suggest that you take the adult fish out of the tank. This way, the fry won’t have to compete with them for the food. However, the adult guppies can be left in the tank if they have sufficient space to move around and look for their own food.

Also Read: What Do Baby Guppy Fry Eat?

3. Conducting Water Changes

Change a third of the water at least once a week. An efficient filtration system cannot keep the tank sufficiently clean. Regular changes won’t stop at simply eliminating the pollutants in the water. They will also combat ammonia spikes along with removing every other toxin plaguing your aquarium.[4]

The suitable water parameters for guppy fry are a pH of 7.0 to 7.2, and a temperature of 80 degrees F. Ammonia, nitrates, and nitrites should be minimal. To monitor those, I use the API Aquarium Test Kit (link to Amazon). That bundle allows me to test the water and adjust the parameters as needed.

As was mentioned earlier, efficient filtration systems and regular water changes are not enough. You have to wipe the walls, scrub the decorations, and remove any waste, leftovers, and debris you find. This prevents the concentration of toxins from spiking. 

Depending on your situation, you can also use conditioners to control the toxins.[5]

4. Avoiding Stress

Guppy fry are just as vulnerable to the side effects of stress as adult fish. Stress can exacerbate the consequences of all the other factors that restrict growth in fish. The easiest way to minimize stress is to maintain a clean tank with the appropriate parameters.

If your fry are still showing signs of stress, you should add plants and decorations if you don’t have them.[6] They provide hiding places. Hiding places put guppy fry at ease, especially if they share their tank with adult fish.

On the whole, you shouldn’t keep young fish with their parents or other adult fish. The bigger fish might eat and chase the fry. However, if you can’t afford a separate tank, the fry can use the plants to evade predators. This will alleviate some of their stress.

5. Providing Enough Space

Some people keep their fry in half a gallon to a gallon of water. But to be on the safe side, especially if you have a lot of guppy offspring, you should get an aquarium of at least 5 gallons. An aquarium of that size will give your guppy fry ample space to explore. 

It is also essential that the tank isn’t overstocked or understocked. Otherwise, the fry will compete with each other over food. That might cause some of the offspring to stop growing. Either that or they will be eaten by adult fish.

6. Choosing the Right Lights

A tank with guppy fry should be illuminated for at least eight hours a day. But the lights shouldn’t stay on for more than 12 hours a day. Also, dim lighting can inhibit their growth, but the lights shouldn’t be too bright either. Improper lighting causes stress in fish.

To avoid complications, I use the NICREW ClassicLED Aquarium Light (link to Amazon). I like that device since it is compact, energy-efficient, and produces a lot of light. It also has a dimmer control to ensure I don’t overheat the guppy fry.

7. Preventing and Treating Diseases

Do not allow diseased fry to remain in the tank. Look for deformities, sores, rotting tails and fins, and the like. Guppy fry are sensitive and vulnerable. Diseases and infections will spread quite easily in their tank. You can quarantine the sick fry, but some aquarists just throw them away because they are unlikely to grow properly.

If the fry share an aquarium with their adult counterparts, do not medicate the entire tank to treat an adult fish.

Even if the drug in question is safe for adult fish, it can still harm younger fish. As was noted before, fry are sensitive and vulnerable. Medication could ruin their health. This is why you are encouraged to quarantine adult fish before you treat them.

How Long Does it Take For Guppy Fry to Grow?

It takes guppy fry six months to reach adulthood. However, they can reach maturity within a few months if you provide them with the right conditions. That includes a conducive environment, suitable water parameters, a high-quality diet, and plenty of space to move around.

Despite the right conditions and the best efforts of aquarists, some guppy fry will still fail to grow, or they will die prematurely. This is often due to genetics or epigenetics. 

Some guppy genes might make them more vulnerable than others. For example, if your guppies carry genes that make them susceptible to a certain disease or infection, you can’t do anything about it.

How Long Does it Take For Guppy Fry to Get Color?

It can take anywhere between one and six weeks for guppy fry to show their colors. In the beginning, they are a dull, unremarkable color. But as the weeks go by, their colors become brighter and easier to identify.

It is essential to provide the fry with the proper diet. Otherwise, they might not achieve the final color of their kind. In most cases, the process of development is faster for fish that have parents with intense colors. That is because they inherit their parents’ genes, which make them more likely to achieve the same colors as their ancestors.

The guppy fry with stronger genes are also more likely to grow faster and achieve a higher level of maturity. But if they don’t get the proper diet, they might not show any signs of growing or die prematurely.

How do You Feed Guppy Fry?

Guppy fry have large appetites and need to eat more frequently than guppies do. They can eat eight times a day. However, that doesn’t mean you have to provide them with eight meals a day. It is enough if you give them three large meals a day, spaced out during the day.

The guppy fry should be fed daily and in sufficient quantities to satisfy their appetites. If you don’t, they won’t grow to their full potential. If you want them to grow as fast as possible, feed them every hour or so. But remember that overfeeding can also stunt their growth by creating too much ammonia in the water.

When Can Guppy Fry Go With Adults?

Guppy fry can go with adults at the age of eight weeks. By this time, they are too big to fit in their parents’ mouths, which is an advantage for them. Also, at this age, they can feed themselves and compete with their siblings for food.

As you now realize, guppy fry are not supposed to share a tank with their parents immediately. However, they cannot stay in the breeding tank forever. As they mature, it will eventually become too small for them.


The process of guppy fry development is rarely easy. At times, it can be frustrating. But once you develop a routine that works for you, the whole process becomes much easier. The important thing is to remember the most important factors that you need to do for your guppies to grow fast and into healthy adults.

These include a suitable temperature of 78 degrees F, a nutritious diet, and a pH of 7.0 to 7.2. Keeping those parameters will mitigate stress, which could stunt the guppy fry’s growth.