6 Common Fish That Will Outgrow Your Tank

Read about six commonly kept fish that often grow larger than expected! Goldfish, pleco, and some catfish need bigger tanks as they age.

Jun 16, 2024byMichael C.
common fish that will outgrow your tank

Setting up a fish tank can be fun and rewarding. However, purchasing the wrong species can produce disastrous consequences! A major problem is that the fish outgrow their tank. There are many fish available at your local pet store that can grow absurdly large. The term “tank-buster” has been coined to describe any of these fish. Let’s highlight five of those species!

1. Goldfish

giant goldfish held on boat
Image credit: City of Burnsville

The first commonly kept tank-buster on the list is the goldfish. Believe it or not, goldfish are surprisingly intelligent––and they can grow almost two feet in length! One popular misconception is that goldfish only grow to the size of the bowl or tank they’re in. This is only partially true. What’s more, being in a small tank can stunt a goldfish’s growth and lead to health problems.

Goldfish are commonly sold as live food for other animals or used as cheap carnival prizes. Don’t let this fool you; provided with proper care, goldfish can live for around 10-20 years on average.

A single common goldfish requires a 100-gallon tank at the minimum. Fancy goldfish don’t get as large as their “normal” cousins, but still require a good-size aquarium to keep them satisfied.

They are also extremely messy fish, producing huge amounts of waste. You need a powerful filter in your aquarium if you want to keep the tank clean.

2. The Leopard Plecostomus (Also Known as Pleco)

plecostomus being held
Image credit: Pinterest

Another commonly sold aquarium tank-buster is the leopard plecostomus, simply known as the “pleco.” Just like goldfish, leopard plecos are often sold in pet stores as juveniles. The leopard pleco can reach up to two feet in length and requires a tank of around 150 to 180 gallons at the bare minimum.

Leopard plecos are popularly sold as “cleaner fish,” which is anything but true. Plecos do eat algae but require other foods in their diet to prevent starvation. Algae wafers and vegetables should all be given as part of a proper diet. Plecos also feed on dead or dying fish.

Like goldfish, plecos are also notorious for producing lots of waste and require strong filtration in their setups. Snails, such as mystery or nerite snails are a great algae-eating alternative to the leopard pleco. It is also important to know that proper aquarium maintenance is key to controlling algae growth.

3. Clown Loach

clown loach school
Image credit: Emma Turner

Known for its playful nature and striking colors (which mimic the much more aggressive tiger barb), the clown loach is an iconic fish that stands out among others. It is also marketed for its ability to eat snails. However, too many people are only familiar with the juvenile loaches, which are marketed as good fish for first-time owners. This fish can reach around eight to twelve inches in size (about the same size as a football), and this social species cannot be kept alone.

Being a schooling species, the clown loach must be kept in groups with a minimum of six individuals. Altogether, a school requires a tank size of around 150 to 250 gallons, with an additional 30 gallons per fish. Loaches such as Botias are smaller and therefore provide much better alternatives than the clown loach.

4. The Oscar

oscar fish schooling in aquarium
Image credit: Chrumps/Wikimedia Commons

Whenever you go to a pet store, chances are that you’ll find the Oscar for sale. Oscars are very popular South American cichlids and come in a variety of patterns and colors, including albinos.

However, some don’t realize that oscars can grow to a foot in length. A single fish requires a bare minimum of 55 gallons per tank. They cannot be kept with any other species smaller than them as they will be eaten. Oscars are also very destructive within the aquarium, ripping apart live plants and rearranging rocks and other objects.

Oscars are also very messy eaters and therefore require powerful filtration. Many people tend to give this fish an improper diet, which leads to health complications such as obesity and fatty liver disease. These fish also thrive in warm water, so you may want to purchase a water heater for your aquarium.

one orange black oscar fish
Image credit: Wikimedia Commons

Oscars should be fed a high-quality pellet and supplemented with a variety of frozen or live foods such as prawns, earthworms, and crickets. Avoid feeding meats such as beef heart. Live feeder fish aren’t needed for your oscar to thrive. Take care not to overfeed your oscars, as they are very greedy fish and will even learn to “beg” for food, even when full.

Provided with proper care, the oscar can be a long-lived pet full of personality (around 10-20 years), but it is very important to do your research before bringing one home (as with all pets).

5. The Silver Arowana

silver arowana swimming
Image credit: Shutterstock

The silver arowana is one fish that a general hobbyist will typically be unable to house properly. With a minimum tank requirement of around 300 gallons for a single animal, the arowana should not be kept by most people. A very strong tank cover is also a must for this species: arowanas can and will jump out if given the chance! Arowanas are also very sensitive to water conditions. They are not a low-maintenance fish by any means.

The silver arowana is often kept as a substitute for the Asian arowana, a close relative. The Asian arowana is currently illegal to keep in some countries (including the United States) due to its protection in the Endangered Species Act (ESA). It is also an important status symbol in some cultures as it symbolizes wealth and good fortune. Because of this, the arowana is a popular species to keep in the aquarium despite its care requirements. It is still not recommended to be kept as a house pet.

6. The Redtail Catfish

redtail catfish and fisherman
Image credit: Acute Angling

The final contender is another example of a wild fish that cannot be kept as a typical pet, at least happily. Like many other tank-busters, redtail catfish are often sold on the market as babies. However, adults can reach lengths of around three to five feet. To house a redtail catfish happily, the bare minimum tank size is around 2,000 gallons (good luck trying to fit one in your living room!).

Housing redtail catfish simply cannot be done for a typical hobbyist and therefore should only be kept by experienced keepers and public aquariums. Some keepers even dedicate large outdoor ponds just for them, complete with proper filtration and heating. Redtail catfish are also territorial, attacking other possible pond or tank-mates living with them.

Unfortunately, the redtail catfish isn’t the only “unkeepable” fish commonly sold as pets. Arapaima, alligator gar, and tambaqui are just a handful of the many other fishes that cannot live happily in a typical aquarium. Even some zoos and aquariums have a hard time housing such animals. Please do your research before choosing to purchase or adopt a fish!

Michael C.
byMichael C.

Michael holds a BS degree in Fisheries and Wildlife from Michigan State University. He formerly worked at a pet store as an animal care associate and is the former president of the MSU Herpetological Society. Michael currently owns three snakes (a corn snake, a Kenyan sand boa, and a checkered garter snake) and a leopard gecko. Interests include almost anything animal-related. Michael enjoys drawing, gaming, and having fun in his free time.