Freshwater monster fish

Monster fish are the tank busters, the arowanas, big catfish, gars, large cichlids and other species that reach 40 cm or more and need custom tanks or ponds. They reward keepers with the space and filtration to match, and punish anyone who buys them small without a plan. Every species here tops 40 cm full grown, so each profile leads with adult size and the tank it really needs. The marine giants, the big angelfish, groupers, morays and unicornfish, sit with the saltwater fish.

Tank size: 53–10,567 gal; 53 gal (200 L) to 10,567 gal (40,000 L); half the list needs 238 gal (900 L) or more
Temperature: 73–82°F; 23–28°C, most of the list
pH: 6.0–7.5; Most of the list
Difficulty: 5 easy; 39 medium · 113 difficult
Temperament: 9 peaceful; 86 mostly peaceful · 62 aggressive

157 species and 6 varieties

Start here

Monsters that fit a large home tank

The ornate bichir is the monster marked easy: 21.7 inches (55 cm) for 172 gallons (650 liters), a slow, armored fish that lives for decades. The clown knifefish reaches 31.5 inches (80 cm) and lists 125 gallons (475 liters), marked medium. The common pleco is sold at 2 inches and reaches 15.7 inches (40 cm) for 85 gallons (320 liters). The silver shark, the bala shark of the shops, reaches the same 15.7 inches (40 cm) and lists 95 gallons (360 liters) for a group. The giant gourami is 27.6 inches (70 cm) of personality for 125 gallons (475 liters). The flowerhorn is marked easy and aggressive at 15.7 inches (40 cm) for 90 gallons (340 liters), kept alone.

The rule

Before you buy a 3 inch monster

Every fish on this page reaches 15.7 inches (40 cm) or more, and the size on the profile is the fish you will own within a few years. The tank is bought for the adult, or the fish is not bought. Filtration scales with the fish: a big predator eats a lot and fouls water faster than a school of tetras ever will. The lid is heavy, because arowanas, knifefish and eels jump. And the plan for the fish's whole life is yours, because shops and public aquariums rarely take a 3 foot fish back.

01
Buy for the adult
The profile's size is the fish you will own
02
Filtration to match
A big eater fouls water fast
03
A heavy lid
Arowanas, knifefish and eels jump
04
A plan for its life
Nobody takes a 3 foot fish back

Arowanas

Arowanas and knifefish

The silver arowana reaches 39.4 inches (100 cm), lists 396 gallons (1,500 liters) and jumps for anything above the surface. The Asian arowana reaches 35.4 inches (90 cm) for 264 gallons (1,000 liters) and is marked aggressive. The black arowana lists 396 gallons (1,500 liters), the African arowana 528 gallons (2,000 liters). The black ghost knifefish is the one most keepers can house, 17.7 inches (45 cm) for 85 gallons (320 liters), a nocturnal electric fish marked difficult. The royal knifefish reaches 39.4 inches (100 cm) for 317 gallons (1,200 liters).

Catfish

The big catfish

The redtail catfish is the classic mistake: 39.4 inches (100 cm), a 1,506 gallon (5,700 liter) minimum, and it eats anything that fits. The tiger shovelnose catfish reaches 35.4 inches (90 cm) for 300 gallons (1,135 liters). The iridescent shark is sold as a 3 inch schooling fish and reaches 39.4 inches (100 cm) for 528 gallons (2,000 liters). The sailfin pleco is the manageable one at 17.7 inches (45 cm) for 100 gallons (380 liters), marked medium. The ripsaw catfish is marked peaceful at 39.4 inches (100 cm) for 396 gallons (1,500 liters). The royal pleco reaches 16.9 inches (43 cm) and eats wood.

Cichlids and predators

Big cichlids, peacock bass and the predators

The jaguar cichlid is marked difficult and aggressive at 15.7 inches (40 cm) for 119 gallons (450 liters). The wolf cichlid reaches 27.6 inches (70 cm), lists 250 gallons (946 liters) and is kept alone. The butterfly peacock bass reaches 29.1 inches (74 cm) for 211 gallons (800 liters) and lives in a group. The red-bellied pacu is the piranha's vegetarian cousin at 34.6 inches (88 cm) for 1,057 gallons (4,000 liters). The Fahaka puffer reaches 17.7 inches (45 cm), lists 119 gallons (450 liters) and is marked aggressive. The wolf fish is a 19.7 inch (50 cm) ambush predator for 106 gallons (400 liters), marked medium.

Ponds

Koi, eels, rays and the pond fish

The koi reaches 23.6 inches (60 cm), lists 264 gallons (1,000 liters) and takes 39–81°F (4–27°C), a pond fish marked peaceful. The fire eel reaches 39.4 inches (100 cm) for 132 gallons (500 liters), marked difficult. The tire-track eel is the smaller spiny eel at 27.6 inches (70 cm). The ocellate river stingray reaches 19.7 inches (50 cm) across and lists 264 gallons (1,000 liters) of floor. The Chinese high-fin banded shark is a pond fish at 26.8 inches (68 cm) for 57–75°F (14–24°C). The channel catfish reaches 39.4 inches (100 cm) and takes 50–86°F (10–30°C).

By the numbers

Smallest

Largest

Easiest to keep

Most peaceful

FAQ

Common questions

What counts as a monster fish?

Any freshwater fish on this list reaches 15.7 inches (40 cm) or more full grown. The profile leads with the adult size and the tank it needs.

Which monster fish are easiest to keep?

The ornate bichir, the common pleco, the silver shark and the flowerhorn are marked easy, and the clown knifefish and giant gourami are marked medium. Easy means hardy, not small.

How big a tank does an arowana need?

The silver arowana lists 396 gallons (1,500 liters) for a fish that reaches 39.4 inches (100 cm), with a heavy lid. The Asian arowana lists 264 gallons (1,000 liters).

Which shop fish outgrow every home tank?

The iridescent shark, the redtail catfish and the pacu are sold at a few inches and reach 34 to 39 inches (88 to 100 cm), with minimums from 528 to 1,506 gallons (2,000 to 5,700 liters).

Can monster fish live in a pond?

The koi, the channel catfish and the Chinese high-fin banded shark are cold-water fish that belong in one. The tropical monsters need a heated pond or an indoor tank.