What to Do When Fish Out Grow the Aquarium?

Fish in your aquarium would not stay the same size forever.

Depending on various factors, they grow to certain expected sizes.

While your fish tank may be the perfect size for your fish initially, what can you do once your fish outgrows the aquarium?

What to Do When Your Fish Have Outgrown the Aquarium?

Logically, there are two things that can be done – upgrade the fish tank or sell or give the fish away.

Upgrade to a Bigger Fish Tank

The logical thing to do is buy a bigger tank so that your fish can fit in it. It is the best possible option for overgrown fish. Fish have their own requirements to survive, which doesn’t include a small tank. Staying in a small tank for too long can cause various health issues and lead to aggressive behavior with other tank mates.

A larger tank will be able to accommodate the existing fish and as well as the new ones if you want to add them.

Think of buying a bigger aquarium as an investment that will act as a beautiful and healthy home for your underwater friends. Larger tanks are also easier to maintain than smaller ones as the temperature tends to fluctuate less.

Other Options

Budget constraints or space issues can cause you to not go for the aforementioned option. If you don’t have enough money to upgrade your aquarium or don’t have enough space to place it somewhere, there are other options you can consider.

In the worst-case scenario, you may have to consider selling or giving away your pets. It is better to do so than to leave them to die in the tank.

If you have enough resources but don’t want to place a large tank in your house, you can try building an outdoor pond for the larger fish members of your pet family.

However, this highly depends on the species of the fish and where you live.

Don’t consider releasing your fish pets in a local lake or canal. Non-native fish can end up having a negative impact on the native fish populations as they end up introducing parasites or depleting their food sources.

Factors Affecting the Size of Your Fish

Cold-blooded animals, including fish, continue to grow in size as long as they live.

They eventually end up outgrowing their homes. However, if you’re wondering how big your fish can grow up to be, you should know that this depends on a variety of factors.


The main factor that determines the size of a full-grown fish is its genetics.

Just like humans grow up to reach different sizes, so do fish. Different fish species can be of different sizes.

While there might be variations in the size of fish in any stated population, individual fish will be close to the average size for their species.

Quality of Diet

Diet is an integral part of the survival and health of any living thing, including fish.

An absence or lack of proteins and vitamins from the diet can cause them to grow at a slower pace and be less active.

Availability of Nutrients

Nutrients are highly crucial for fish to thrive.

If they aren’t provided with enough nutrients, fish can become more prone to injuries due to weakness.

They will also heal at a slower pace, become more susceptible to disease and be less tolerant towards aggression from fellow aquarium mates.

Cleanliness of Aquarium

Ensuring that your aquarium remains clean is an integral part of ensuring that your pet fish remain healthy and happy.

The aquarium should be kept clean and should have good quality water; otherwise, the fish face a high risk of organ failure. Since the cleanliness of the aquarium is a huge factor that can cause health problems, it also affects the size of the fish.

Impact of Hosting Fish in a Small Tank

Stuffing too many fish in a small tank will result in a cramped up house party. While your tank might seem colorful and entertaining initially, everything will head downhill really soon.

When fish don’t have enough room to grow or move, not only are they unable to thrive but they also face other problems that cut their lifespan short. If you doubt the size of a fish tank, always remember – the bigger, the better!

Here are some problems that your fish can face if you don’t place them in a bigger tank.

Reduced Life Span

A young fish growing up in a significantly small tank is highly likely to experience stunted growth, leading to spinal deformities, muscle atrophy and a world of other health problems.

Additionally, stunted growth tends to shorten the lifespan of the fish too.

If you’re aware that the fish species you just purchased will outgrow your aquarium, return it or choose to upgrade its aquarium while it’s young.

Once the health issues start occurring evidently, it will be difficult to find a new and appropriate home for it.

Fighting for Space

Imagine sharing a single room with another person for the rest of your life. While it might be entertaining and fun being roommates for a time, eventually their presence will start getting on your nerves.

This annoyance will cause a rift between you and the other member. This is exactly what occurs when too many fish, particularly territorial species, are thrown in and forced to coexist in a tiny space.

Fish require an ample amount of space to hide or get away from other tank mates. If they don’t get that, you can expect to see a lot of aggression and issues within the tank.

Not only should you ensure that your fish pets have enough space for themselves as they grow, but you also need to make sure that the species you’re hosting are compatible.

For this, you should consult a local fish expert to be aware of the species of fish you can and should inhabit in your tank.

Poor Water Quality

When it comes to water, the nitrogen cycle is a highly integral part of maintaining a healthy aquarium.

The waste that fish expel turns into ammonia and the bacteria change the ammonia to nitrite. This nitrite is turned into a less-toxic nitrate.

Stuffing fish into a small aquarium can result in extreme havoc on the water quality. To prevent this, you will have to change the water, vacuum the gravel and constantly keep up with the waste bioload.

Additionally, you will also have to maintain the feeding patterns of all the fish in your tank to prevent excessive waste. However, failure to do so will result in poor water quality, which will eventually lead to death.

Stress and Illness

Fish, by nature, were created to move around a river, ocean or pond. They naturally require a large habitat with enough space that allows them to move around.

Offering them a limited amount of space in comparison to their size can cause stress and cabin fever. A healthy fish is happy in its environment, has a stronger immune system and can fight off diseases.


Fish are born to be free creatures and if you choose to make them your pet, you must ensure that they have ample amount of space to live and thrive in peace.

You can use an aquarium calculator to determine the fish population in your tank and if you have enough space to add new fish to your tank.

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