10. Goldfish
Goldfish are often thought of as small fish that live in bowls, but they can actually grow much larger than many people realize. With proper care, goldfish can reach lengths of up to 30 cm (12 inches). These “stealth tankbusters” frequently surprise their owners by outgrowing their tanks and needing larger ponds to thrive.
9. Clown Loach
Sold just a few centimeters long, Clown Loaches can grow into impressive giants in home aquariums. With a lifespan of over 20 years, these vibrant fish can reach sizes of 30 cm (12 in) or more. They thrive in groups and require spacious tanks to support their growth.
8. Bala Shark
Bala Sharks have a misleading name; they are not actually sharks. These fish can grow to about 35 cm (14 in) or more. They are fast swimmers and like to school together. Because of this, they need large tanks with plenty of space to swim.
7. Tiger Oscar
Tiger Oscars start small but quickly outgrow most home aquariums, reaching lengths of 45 cm (17.7 in). Highly intelligent and interactive, they recognize their owners and can even learn tricks. However, they can be aggressive, making them best suited for large, well-maintained tanks.
6. Plecostomus
Plecostomus, commonly referred to as “plecos,” are often purchased as small algae eaters, but they can grow surprisingly large—up to 60 cm (24 in). These armored catfish need spacious tanks and pieces of driftwood for hiding. While they are effective at keeping tanks clean, their substantial size can take unsuspecting owners by surprise.
5. Koi Fish
Koi fish are not only decorative inhabitants of ponds; they can grow up to 90 cm (36 inches) in length! With proper care, these stunning fish can live for several decades, with some even exceeding 50 years. Their vibrant colors and graceful movements make them a favorite choice for outdoor ponds.
4. Arowana
Arowanas are highly valued for their dragon-like appearance and impressive size. They can grow up to 120 cm (48 inches) in length and are known for their powerful jumps. These predatory fish require large tanks or ponds to thrive, and their aggressive nature can make them challenging for novice fishkeepers.
3. Red-Tail Catfish
The Red-Tail Catfish starts small but can grow into a true river monster, reaching 150 cm (60 in). Found in South American rivers, they are powerful predators with insatiable appetites. Many aquarium owners underestimate their growth, leading to them being rehomed in large ponds or public aquariums.
2. Alligator Gar
The alligator gar is one of North America’s biggest freshwater fish, reaching 240 cm (96 in) long. With its prehistoric look and sharp teeth, it’s a powerful ambush predator. It thrives in slow waters and can live for decades, often outgrowing home aquariums.
1. Arapaima
At the top of the list is the Arapaima, a giant freshwater fish native to the Amazon. This massive creature can reach lengths of up to 270 cm (108 inches) and weigh over 200 kg (440 pounds). As one of the largest bony fish in the world, the Arapaima surfaces for air every 10 to 20 minutes, making it a remarkable and intriguing predator.
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