How Long Does it Take for Baby Fish To Grow?

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Not all fish species reach the same sizes or grow at the same rate. Some mature faster, while others take their time, and knowing your favorite species’ growth cycle is essential to provide it with optimal care. 

As a general rule, fast growers are also proficient breeders, making them ideal for fish selling businesses and selective breeding. On the other hand, slow growers take more time to mature and will produce fewer offspring at longer intervals.

I recommend investigating each species’ profile before acquiring it to prevent unwanted scenarios where you end up with more fish fry than you can handle. To lend you a big and strong hand, here are the growth rates of 20 popular fish species that you should consider for your aquarium:

Guppy

Adult sizeGuppy females grow larger than the males, reaching around 2.5 inches as adults. Males will remain smaller, between 0.7 to 1.4 inches. The latter will display slightly longer and more colorful bodies to make up for their lackluster size.

Growth rate – Guppies actually grow slower than you might expect. They will reach adulthood at 6 months of age, and the fry typically grow at a consistent pace. All parameters being equal, guppy fry will put on .3 inches per month. You can boost their growth rate a bit by providing high-protein meals and ensuring higher water temperatures, whereas colder waters will inhibit their growth.

Molly

Adult size – Mollies differ in size, varying between 3 to 5 inches. They rank as rather small fish and require tank mates similar in size to prevent bullying or predatorial behavior.

Growth rate – Mollies also differ in growth rate, usually depending on their environmental parameters. They will generally reach their adult size within 4 to 6 months but need more to become sexually mature. Male mollies will reach their sexual maturity at about 1 year of age.

Oscar

Adult sizeOscars are big fish, capable of reaching 11 to 12 inches as full adults. Some Oscars may even reach 16 inches and weigh over 3 pounds, depending on their diet, environmental factors, and genetic makeup. An average-size Oscar, measuring 10-11 inches, requires 55 gallons of water to remain comfortable, healthy, and active throughout the years.

Growth rateOscars grow faster than most tank fish. They will pack on 1 inch per month for their first year and take approximately 12 months to reach their adult size. This growth rate is impressive by tank fish standards, and many novice Oscar keepers don’t see it coming. This may force them to upgrade their tank soon after acquiring their Oscar.

Discus

Adult size – The Discus fish can reach sizes of up to 8-10 inches, depending on nutrition and habitat conditions. This is the fish’s size in diameter since Discus fish are pretty much round in shape.

Growth rate – This species has a slow growth rate. The Discuss will reach adult size at the 2-2.5-year mark, at a rate of .5 inches per month. This is great if you want a larger fish species that grows slower, so you don’t have to upgrade your tank constantly.

Gourami

Adult size – Adult gourami will grow up to 5 to 6 inches. Adequate care, impeccable water conditions, and stable parameters will boost their growth significantly, helping them grow faster and larger. The diet also plays a major role in the fish’s development. There are, however, major differences in the size of many gourami species. Some will grow up to 8 to 10 inches, while others, like the giant gourami, can reach 28 inches.

Growth rate – Slow growers. Your typical gourami may grow up to 0.5 per month, but it strongly depends on conditions like water parameters, available space, and food. You can provide the fish with more nutritious and consistent meals and provide them with increased space to boost their growth.

Corydoras

Adult size – Corydoras rarely grow more than 2 inches as adults. Females will be slightly larger, like in most fish species, reaching 2.5-3 inches, almost identical to guppies.

Growth rate – Corydoras aren’t your typical fast growers, but that generally fluctuates. Expect the fry to double its size within the first year and slow down its growth rate after the first 12 months. However, you can influence your Corydoras’ growth rate. Frequent water changes, stable environmental parameters, and a diverse diet allow Corydoras to grow faster and reach larger sizes.

Bristlenose Pleco

Adult size – Despite being a catfish, the Bristlenose Pleco only grows around 20% of what a normal catfish would. An adult Bristlenose Pleco grows up to 3 to 5 inches. By comparison, other species of catfish grow up to 22 inches, with 52 inches being the maximum recorded.

Growth rate – The Bristlenose Pleco is a very slow grower. It will typically remain around 1 inch during its first 3 months of life and add another 2 to 3 inches for the following 3 months. This species reaches sexual maturity at around 6 months of age.

Platy

Adult size – Platies can reach a size of up to 3 inches under peak environmental conditions and adequate food. These are colorful and active fish, thriving in community tanks and coming in a variety of colors. Provide them with optimal water parameters and a complete and diverse diet, and they will remain healthy for years to come.

Growth rate – These are fast growers, capable of reaching their full adult form in about 4 months. This is astounding compared to other tank species that display a slower growth curve.

Swordtail

Adult sizeSwordtails are distinct-looking fish that can reach sizes of up to 5 to 6.5 inches. The lower value describes the males, which are known to stay slightly smaller than the females. The latter will usually pack on at least an extra inch.

Growth rate – Swordtails undergo several growth stages, including fry, juvenile, adult, and old. The fish will grow around 0.6 to 0.8 inches per month during the first 2 phases, then slow down a bit for the following months. Swordtails will reach their adult size (5 to 5.5 inches) 6 months after birth. With proper care, you can boost their adult size a bit.

Angelfish

Adult sizeAngelfish need no introduction. These exquisite fish can reach sizes of up to 6 inches in captivity. Their fins will be even larger, around 8 inches on average, providing the Angelfish with a distinct look.

Growth rate – The Angelfish will grow rapidly within the first 6 months of its life. Expect the fish to increase in size by around 4 inches. After that period, its slow rate will decrease dramatically, even with optimal living conditions. The Angelfish will only reach its adult size at around 1 year of age.

Tetra

Adult size – Tetras are small, thin, and shaped like torpedoes and will reach around 1.5 inches as adults. They might grow slightly larger than that, but not by much. In the wild, however, various species of tetras can double that value.

Growth rate – Tetras grow slow, so they will remain tiny for a long time. Your average tetra will only grow up to 0.25 inches within its first 2 months of life, and it doesn’t get any better after that. Depending on environmental conditions, diet, and species’ profile, you should obtain a fully-grown tetra within 8 to 12 months.

Rasbora

Adult size – Rasboras are small and fun fish, coming in a variety of colors and patterns. They will typically grow up to 4 inches but won’t exceed 2 on average. This makes them ideal for community tanks with other friendly and peaceful species.

Growth rate – Rasboras are some of the fastest growers in the aquarium business. The fish will reach its adult 2-inch size in about 3 months after its birth. It is also when they become sexually active if you plan on breeding them in mass.

Betta

Adult size – The Betta will only grow up to 3 to 4 inches as an adult fish, although it will usually appear a lot larger. This is due to its excessively large fins and tail, making the Betta appear larger.

Growth rate – The Betta will grow up to 2 inches within its first 3 months of life. The fish should reach its adult size within the first 7 months, but that can differ. Some Bettas grow faster under optimal environmental conditions.

Otocinclus

Adult size – This is a small fish, only capable of reaching up to 1.5 to 2 inches. However, this species’ high energy levels will make up for its small size. Your Otocinclus fish will dart around its environment like it’s no one’s business.

Growth rate – The Otocinclus grows slow, reaching its full size about 5-6 months after birth.

Danios

Adult size – Most species of Danios will reach around 2 inches, while several others can grow more than double that. Danios fish are generally small and active and make for great community fish.

Growth rate – Danios are moderate-to-fast growers. Expect your Danios fish to grow 1 inch within their first 6 weeks of life.

Rainbowfish

Adult size – The rainbowfish will display a variety of colors and sizes, depending on the subspecies. Most species of rainbowfish will only grow up to 4-4.5 inches, while others will barely reach half that. Then you have species like the Melanotaenia Vanheurni, capable of putting on 8 inches.

Growth rate – Expect your rainbowfish to reach its full size approximately 12 months after birth. Some might take longer, depending on the species. Provide the rainbowfish with a more diverse diet and the fish will improve its growth rate visibly.

African Cichlid

Adult sizeAfrican cichlids can vary in size considerably, capable of reaching around 3-5 inches on average. However, some species may grow larger, sometimes up to 8 or even 10 inches. This means that not all African cichlids can adapt to the same environment, requiring personalized living conditions.

Growth rate – Most African cichlids can grow up to 1 inch per month during their first 4-5 months, after which they slow down a bit. But their growth rate depends on a variety of factors, including the species (there are over 1,600 species of African cichlids), food, and environmental conditions.

Killifish

Adult size – These fish remain small, around 1 to 2 inches for most species. Some species can even reach 6 inches in the wild, but that’s rare in captivity.

Growth rateKillifish are some of the most proficient growers in the vertebrate world. This species can reach sexual maturity within its first 3 weeks of life, which is also when they typically reach their adult size. One species in particular, the turquoise Killifish (Nothobranchius furzeri) in Mozambique, can achieve sexual maturity within 14 days.

Goldfish

Adult sizeGoldfish’s size can vary dramatically depending on its environmental conditions. The fish can reach 12-15 or even 20 inches in the wild. Some suggest even more than that. In captivity, its maximum size is influenced by the size of its enclosure and habitat conditions. The goldfish will remain around 1-2 inches as an adult in small tanks. The larger the habitat, the bigger the fish will grow. Some breeders have reported that their pond goldfish have grown in excess of 12 inches.

Growth rate – The goldfish will typically reach its full adult size within the first year, but that’s not a universal rule. Expect your goldfish to put on around 2 inches within the first 6 months, 3 by the end of the year. Proper nutrition, a larger living space, and optimal water parameters will boost its growth rate and maximum size.

Barb

Adult size – Barbs won’t grow more than 2.5-3 inches on average. Wild species can reach 5-6 inches in optimal conditions, which you can replicate in captivity. If you’re lucky, this approach should boost your barbs’ maximum size by 1-1.5 inches.

Growth rate – Barbs tend to grow around 0.5 inches every 6 weeks, but this isn’t set in stone. Tweak the fish’s tank parameters and improve its diet and you might be able to get more out of it.

Will Baby Fish Survive in My Tank?

The answer is fully dependent on how you care for your fish. It all begins with realizing that tank fish don’t usually exhibit any relevant parental instincts. Not most anyway. The majority of aquarium fish leave their fry soon after birth, especially livebearers like guppies.

They will also eat them given the opportunity since they can’t differentiate between their own offspring and their regular prey. With this in mind, you should adopt several preventive measures to improve your fry’s survival rate. These include:

  • Preparing a breeding tank – Don’t let your fry spawn in the same tank with the adults. Monitor the female(s) pregnancy and prepare a breeding tank as soon as the labor approaches. You can usually tell when the moment comes by assessing the female’s body and behavior. If the female displays a drastically increased belly with a dark gravid spot, refuses food, and tends to swim in place, it’s on. Move it into the breeding tank and wait for the fry to arrive, then move the female back into the main tank after delivery is complete.
  • Set up the breeding tank accordingly – Ensure proper filtration, but make sure the filter isn’t too powerful to suck the fry in. Sponge filters are a great option. You should also set the tank’s temperature slightly higher than adults’ since the fry needs higher temperature to optimize their growth.
  • Perform regular water changes – Fish fry require more often water changes, depending on the species. Keep their water clean, monitor oxygen levels, and prevent overfeeding to avoid elevated ammonia levels.
  • Relocate them to the main tank – For small fish species like guppies, the fry are ready to go into the main tank when they reach 1 inch in size. Larger fish species may need more time to grow, depending on their adult size.

These strategies should increase your fry’s survival rate significantly.

How to Make the Baby Fish Grow Faster?

The good news is that you can boost your baby fish’s growth rate beyond what they’re naturally inclined to. The unfortunate news is that you need to adopt several strategies for that. And the even better news is that they’re easy to apply. Case and point:

– Choose the Right Tank Size

Most fish species will grow bigger in larger environments, but how large the environment should be depends on several factors. It depends primarily on the fish species and how many fish you have. Some fish species require more space then others.

Guppies, for instance, need around 2 gallons of water per fish. Oscars, which are way larger and grow a lot faster, may require 55 to 75 gallons as adults per fish. Provide the baby fish with the necessary space to grow and consider their growth rate and space needs in the process.

– Provide Good Diet and Food

Fish fry have almost identical diets to their parents with some slight variations. The fry’s growth rate is significantly faster, which means they require a lot more protein during their first weeks of life. They may also need more frequent meals than adult fish.

Tweak their diet accordingly, provide them with varied meals, and they will grow faster and bigger, all things being equal.

– Regular Water Changes

Baby fish are more sensitive to water parameter changes. Since they eat more than the adults, they will also poop more, causing their environment to degrade faster. Ammonia spikes will hurt them, so keep their water clean.

You may need to perform 25% water changes every 3-4 days during their first several weeks of life. It also depends on how many there are in the breeding tank.

– Reduce and Eliminate Stress

A lot of things can stress out the fish, including bullying from larger fish and even sudden movements, noises, and lights in the tank’s room. Fish stress can cause weaker immune systems, at which point your fish will become prone to infections and parasites.

Provide your baby fish with a lot of hiding places to prevent that. Have some plants, several rocks, wood, and maybe a cave to help them feel comfortable and safe.

– Keep Stable Water Parameters

Ammonia spikes and elevated levels of nitrites can kill fish fry no matter the species they belong to. Regular water changes and water parameters monitoring can prevent this problem and keep the fish safe.

You should also minimize any water parameter fluctuations like pH, hardness, and temperature. Keep in mind that the fry require higher temperatures to optimize their growth compared to adults.

Conclusion

The factor that influences the fish’s size and growth rate the most is their species. Guppies will never exceed 2.5 inches, with some extreme exceptions and not by much, while Oscars will rarely remain under 9 inches.

Learn your favorite species’ profile, set up the tank adequately, and ensure the fish with all the necessary conditions to thrive. And most importantly, the fry always require different environmental conditions and diets compared to adults.

avatar I’m Fabian, aquarium fish breeder and founder of this website. I’ve been keeping fish, since I was a kid. On this blog, I share a lot of information about the aquarium hobby and various fish species that I like. Please leave a comment if you have any question.

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