How Long Does It Take A Fish To Grow To Full Size?

When people first get a fish, they are usually small juvenile fish.  Sometimes these small fish will eventually grow to become very large fish. Over time with the correct feeding, lighting, and care these fish will reach a full-size adult fish. 

How Long Does It Take a Fish to Grow to Full Size? The length of time it takes a fish to reach its full size depends on many different variables, but usually, in about 1-2 year, your fish should be the adult size. For example, a Guppy fish can reach full size within about six months whereas a Plecostomus may continue to grow on up to its life expectancy of ten years or so. 

Can the Size of the Tank affect the Size of My Fish?

Many fish keepers want their fish to hurry up and grow so they can become the beautiful adult fish you see in commercial aquariums. What some don’t realize is the 5-gallon tank you started with for your baby fish may not be sufficient when that fish gets big. It is important to plan ahead and have your tank set-up for the size your fish will get not for the size it is right now. Without the appropriate room to move around your fish simply will not thrive as well. 

Contrary to some superstitions that a fish will only grow as big as it’s environment will allow; fish will continue to grow until they literally can’t move. The rate at which they grow may be hindered because they can’t swim around and build healthy muscle, but they will still in fact grow. 

Goldfish keepers usually go by the rule of at least 5-gallons per fish. Goldfish and Koi can grow to be very large and sometimes even that rule needs to be adjusted. 

They can live a very long time, read “ which pet fish live the longest” and find out how old the oldest pet fish ever lived! You’ll be amazed.

So how long does it take to reach maximum growth?

As stated before, there are approximate lengths of time for every species but the rate at which any fish will grow will greatly depend on the quality of husbandry.

Say you have a baby Swordtail. Most Swordtails can reach maximum size in about 2 years. But if the water quality and feeding schedule and lighting are all sub-par then that number will be skewed somewhat. Keeping proper water, food, and lighting parameters will help tremendously with the growth of any species. 

Basically, the better your husbandry the faster your fish will grow and the healthier it will stay therefore enabling it to reach its full potential in a timely manner. 

How Long for My Small Fish to Reach Adult Size?

Another thing to consider if you want your fish to grow into adulthood quickly is the size of the species. Most smaller fish like guppies, tetras, and angels can reach an adult size typically within a year. Again, that is an adult size, not exactly their full size but close enough to gauge how big they will ultimately be. 

So obviously the smaller the fish species is the faster it will reach its full potential because its potential is still relatively small. But for most people that keep smaller species, they tend to keep more than one so even though a neon tetra may not be very big full grown if there are fifty of them the tank still needs to provide the appropriate room for them to swim around without bumping into each other all the time.

How Long for My Large Fish to Reach Adult Size?

There are large species that grow very quickly though. The Arowana, when compared to most fish kept in a home aquarium setting, is considered to be gigantic and can reach its colossal size in a relatively short amount of time. Within the first year, Arowana’s can grow 2 inches a month. That equates to 2 feet long in one year with a maximum size of 3-4 feet long weighing 10 pounds or so. That is a massive aquarium fish and is in no way suitable for a beginner aquarist.

Fish like the Arowana have become very popular because of their size and the beautiful pearl silver scales along their bodies. But unfortunately, the rate at which they grow is also their downfall.  Ignorance leads them to be purchased without the buyer realizing just how fast and how big they actually get and results in them being dumped in rivers and lakes to fend for themselves.

Arowanas grow fast
Arowanas grow exceedingly quickly. Too fast for most aquarists to house

Although, sometimes they thrive which can lead to devastating consequences on the environment. Snakehead fish are a shining example of what happens when a non-native species is introduced into a delicate ecosystem.

Be Prepared for the Size Fish You Are Going to Have

So this leads to the importance of knowing your fishes potential full size and being prepared for it. Also, it is important to keep in mind the number of fish you are going to want. Have you ever seen a baby Tiger Oscar Fish? Cute, hey. But just wait 2-3 years and see how big they get and how fast they grow. A good pet store or aquatic retailer should advise you on the best fish to keep in relation to your aquarium water volume and give you an idea of how long it will take for that fish to become fully grown and become an adult fish.

Conclusion: How long does it take for a fish to become fully grown?

It seems as a general rule most fish are going to reach a respectable size within about a year or so again depending on the species. For any fish keeper though it is the excitement of seeing your new baby fish grow and change that keeps their interest.

You can go all nerdy and look at all the scientific data on why and how long it takes fish to reach maximum size.

But most people don’t really care how fast they grow as long as their fish are beautiful and healthy. There are many variables when answering the question of how long does it take for a fish to grow to full size but as long as the environment they live in is kept in good standing then you can rest assured that eventually, they will get there no matter what size or species they are.

For more help in buying the right size aquarium and setting your first aquarium up correctly read our beginners guide to starting a fish tank.  

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