Cute now, big later: 6 freshwater fish that can take over your aquarium

Aquariums have become a big trend lately. More people are buying small fish and setting up tiny tanks at home because they look cute, peaceful, and are genuinely relaxing after a long day. But there’s one thing many beginners often overlook.

Many aquarium fish are sold as babies, which means they start out very small. However, some species grow quickly and can become surprisingly large. This is when problems start, as the fish suddenly need a much bigger tank, or sometimes even a pond.

Many first-time hobbyists only realise this once their fish have grown too big, making it difficult to manage space at home. Larger freshwater fish eat more, need plenty of room to swim, and often behave differently, which is why experts usually recommend that beginners avoid them.

Here’s a list of freshwater fish that may look tiny and adorable in shops but can grow into massive adults.
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Disclaimer: The information in this article is for general guidance only. Aquarium conditions and fish growth may vary depending on the environment, care, and other factors.

Images: Canva (for representative purposes only)

Arowana has a long, sleek body and swims very gracefully. Often called the dragon fish because of its appearance, it starts small but can grow up to around 3 feet. It’s a strong jumper, so the tank needs a secure lid. Arowana eats meat and can be territorial, making it challenging for beginners to keep in a regular home aquarium.

True to its name, the Giant Gourami can grow up to 24 inches and needs plenty of swimming space. Native to Southeast Asia, it can even recognise its owner. Because of its size, it’s usually kept in large tanks or outdoor ponds.

Pacu resembles a Piranha but is mostly peaceful and vegetarian. It can grow up to around 30 inches. Many first-time hobbyists buy it when it’s tiny, only to be surprised by its size later. Due to its growth, Pacu is often abandoned when it outgrows a normal home aquarium.

Redtail Catfish is easy to spot thanks to its bright red tail. In captivity, it can grow over 4 feet, which is huge for any indoor tank. While gentle, it grows fast and will eat anything that fits in its mouth, including other fish. That’s why it’s best kept in very large tanks or ponds.

Oscar fish are popular for their vibrant colours and playful behaviour. Larger varieties can grow between 14 to 16 inches, requiring lots of space. They need clean water and can become territorial as they grow, which can be tricky for beginners.

Koi fish come in many beautiful colours and look small when young. As they grow, they become very large, so they are mostly kept in outdoor ponds rather than indoor tanks.