Beware of the Tank Busters: 7 Fish that will Grow Too Big for your Aquarium

A tank buster is any fish that is likely to outgrow even the largest tanks available to the average everyday fish keeper, or indeed, the space in the home available to site such a tank. Many argue that this is an ethical and welfare issue; that these fish do not belong in the regular aquatic trade and should only be available by special order to expert and experienced keepers.

Unfortunately, many larger species are particularly attractive when young, they are often easy to acquire for resale, and some aquarist stores are less than honest when selling these fish to buyers. Here are some of the most common fish you should avoid, if you want to prevent ending up with a giant fishy problem on your hands.

Oscar Cichlids

The Oscar cichlid is another common species, and very popular being an attractive fish with a lot of personality. They are easy to breed and fry are easy to raise as the parents practice parental care. The tiny babies are very appealing being mottled all over. But not only are they unsuitable for the community set up, being voracious predators which will eat anything they can get in their mouths, they are destructive, moving and damaging tank furnishings, and they grow. They can reach 35 cm, and being very active fish, are felt to do best in a spacious tank of at least 125 gallons. They also live about 10 years.


Often sold as algae eaters, Plecos (Plecostomus) come in several varieties and are bottom feeding catfish with sucker mouths that are of striking appearance and very easy to acquire. Owners should be aware that they require additional feed as their algae eating tendency often lessens as they grow and they should never be expected to survive on that alone. Most species also need big tanks, as those sold under the trade label Common Pleco can easily attain a length of 30 cm.

Red Tailed Catfish

Very cute when little and often sold at about 15 cms, these fish are cheap to acquire and pretty to look at, with their creamy white and black markings and red accent to the tail. Although not aggressive, they are voracious eaters and will devour any fish they can fit in their mouths. They grow incredibly rapidly, and to about 134 cm. Whilst undoubtedly handsome fish and full of personality, only a well-informed expert with a super-sized aquarium should take one on. Most purchasers will face a large and lengthy future problem; these huge fish have a 20 year lifespan. (Read here our full Red Tailed Catfish breed profile)


Another attractive fish which is all too easy to acquire, the Pangasius is a silvery black grey and white catfish often sold under the popular name of Iridescent Shark. They are active and elegant swimmers and require a lot of space for this, and being easily stressed if isolated should be kept in shoals, so already need a larger tank, even when small. They also grow to enormous sizes, about 130 cm, and like the Pacu can be very nervous, so if panicked could crash into and break the glass of their tank once they reach sufficient size.

Clarias Catfish

In the aquarium trade the Clarias or ‘walking’ catfish comes in some attractive marbled or albino varieties which are usually the reason it is sold to unwary pet owners. It gets its alternative name for the fact that it can use its stiff pectoral spines to walk across land to fresh pools of water in the wild- it may escape the tank using these appendages and requires a firmly secured lid. These fish are highly predatory and grow rapidly, so even fish that were too big for them to eat when they first entered the aquarium may not stay safe for long. This fish often reaches 60 cm in captivity but can grow as big as 100 cm, and could easily require a tank of around 500 gallons.


Pacus are quite commonly available in the pet trade and often bought as tiny, cute little silvery fish about 5-8 cms in length. Unfortunately, their growth rate is extremely rapid and as they can reach about 110 cm their owner may quickly find even the biggest tank they can fit in their home is not big enough. Even worse, they are shoaling fish and should be kept in groups, increasing the space required, and being nervy they can charge into the glass at full speed if alarmed – these have the very literal potential to be tank busters. Finally, this is not a problem that will just go away. Pacus can live for up to 40 years.

Arowana, the dragon Fish

These predatory fish, also known as dragon fish, are growing in popularity and becoming more widely available. Mature specimens of the rarer and specially bred colour varieties command high prices. They are often sold as about 8 cm long babies with their egg sacs still attached. Most species can reach 120 cm and as they are best kept in small groups their requirements will soon require more space than can be provided by the average home fish keeper.

The best advice is not to acquire these fish as appealing small juveniles if you know you will be unable to accommodate their substantial adult size. Resist all arguments – some unscrupulous sellers will try and persuade you that it takes so long for them to grow that there is no need for concern, or that you can bring them back – in fact, the problem can become real astonishingly quickly and few vendors will follow through and take back such a large fish.

The commonly quoted answer of donating the fish to an aquarium or zoo cannot be considered an automatic solution – these places too have limited space. Avoid the problem altogether, walk away and leave these fish in the seller’s tanks.

My name is James, and I’m in love with aquariums and fish since I was 12 years old. Back then my dad gave me a goldfish, and it’s been 35 years learning about this fascinating hobby. I’ve had some freshwater aquariums, tried my hand with marine tanks for 10 years, and kept some reefs for a while too. Here in the website I try to share some of my knowledge and experiences on fish keeping.


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