Whale Catfish (Cetopsis sp. 'Orinoco')

Choose a Whale Catfish only for a species tank

The Whale Catfish sold as Cetopsis sp. 'Orinoco' is a member of the Cetopsidae, the whale catfishes of South America. FishBase describes the family as whale-like catfishes with a naked, scaleless body, three pairs of barbels and no adipose fin; the name Cetopsis means whale-like. They are smooth, slimy, torpedo-shaped fish with small eyes, built for swimming in the open water of large, flowing rivers.

We want every buyer to know exactly what this fish does. The best-known member of the genus, Cetopsis coecutiens, which lives in the Orinoco, is a scavenger and opportunistic predator. Seriously Fish describes it swimming in voracious feeding groups in the wild, homing in on weakened fish by smell and burrowing into prey, and says it will bite chunks out of tank mates much bigger than itself. It is a remarkable oddball for a specialist with a species tank, not a fish for any community aquarium.

Check the facts before you buy

Understand the name Cetopsis sp. 'Orinoco'

'Orinoco' here is a trade label for where the fish was exported from, not a confirmed species. More than one whale catfish lives in the Orinoco basin, and young ones are hard to tell apart. Planet Catfish notes that C. coecutiens is an uncommon import and that several other members of the family are imported rarely. These are the two named species most relevant to this label:

Compare the likely species

Because the species cannot be guaranteed, this page plans for the larger, better-documented C. coecutiens, and all care values below come from that species. If your fish stays small and slim, it may be one of the smaller species; the care principles are the same. One purchase quantity supplies one live fish in a single Standard option, because no size grade is recorded. The photographs show the type of fish, not the individual you receive, and gallery images are representative.

Recognise a whale catfish

• Body: cylindrical, torpedo or cigar-shaped and naked, without scales (Planet Catfish).
• Eyes: small and "piggy"; Seriously Fish explains they are covered by a thin layer of skin so they are less likely to be damaged when the fish burrows into prey.
• Skin: particularly slimy, which helps it push into prey.
• Mouth: large, with the lower jaw underslung and the upper jaw protruding, so the fish seems to smile.
• Colour of C. coecutiens: blue on the back and head, white below, with some blue in the dorsal and tail fins.

Plan a long tank with room to swim

Whale catfish are midwater swimmers that keep moving. For an adult C. coecutiens, Seriously Fish gives a minimum of 120 × 45 × 45 cm, about 243 litres. Chris Ralph, who caught these fish in Peru and wrote about them for Planet Catfish and ScotCat, suggests 72 × 24 × 24 inches, about 183 × 61 × 61 cm, to keep them to adult size. We recommend the larger figure if you plan a group. Use our tank volume calculator to check the real volume of your tank.

Run a four-step space check

1. Measure the base: at least 120 × 45 cm, and longer for more than one fish.
2. Keep the middle open: this fish needs swimming room, not clutter.
3. Fit a secure cover: OATA advises fish-proof covers for large, active fish.
4. Plan for no other species: companions are at risk, as explained below.

Set up a sparse, dim, fast-flowing tank

Seriously Fish says décor is not critical but water movement is: position the filter return so it breaks the surface, or add a powerhead at one end. It also recommends dim lighting, because the species' natural waters are usually deep and murky and it is much more active in low light. Planet Catfish agrees: a sparse, dimly lit tank with only minimal cover, and sand or rounded gravel on the bottom because the fish often comes to rest there.

Divide the footprint by function

Planet Catfish suggests a dim actinic or night light so you can watch its nocturnal prowling. Keep sharp décor out: a scaleless fish damages easily.

Hold the water values steady

The sources agree closely for C. coecutiens, so the target is simple.

Check your tap water against the target

This is a forgiving range. Soft water from much of Scotland, Wales and the north-west usually fits, and so does moderately hard water; only very hard, alkaline tap water may push pH above 7.4. Test your supply and use our water matcher to compare it with the target, and see our water chemistry guide if you need to adjust it. Seriously Fish lists this species as needing a steady temperature, so use a reliable heater and keep the tank away from draughts and sunlight. Keep ammonia and nitrite at zero and nitrate no more than 20 mg per litre above tap water, as OATA advises for catfish.

Filter for flow and change water often

Planet Catfish describes the whale catfish as an open-water fish that needs strong current and good oxygenation, and Seriously Fish asks for a good degree of flow through the tank. Choose a filter rated well above the tank volume, aim its return to break the surface, and add a powerhead if needed. Our guide to aquarium filtration explains how to size it, and our guide to the nitrogen cycle explains why the tank must be mature first.

Follow a weekly routine

1. Test ammonia, nitrite, nitrate and pH every week.
2. Change at least 25% of the water weekly, as Chris Ralph recommends for this meat-eating fish and OATA advises for catfish.
3. Siphon every scrap of uneaten food; a greedy meat-eater fouls water quickly.
4. Rinse filter media in removed tank water, never under the tap.

Feed meaty foods, and never overfeed

Seriously Fish says there is no need to provide live fish because it is an eager and aggressive feeder: offer frozen and live foods such as prawns, mussel, cockle, lancefish and earthworms, and some individuals even accept dried foods. Planet Catfish names chopped mussel and live earthworms as favourites, warns against weaning it onto feeder fish, and advises trying shrimp-based tablet foods from a young age.

Build a measured meal

Both Seriously Fish and Planet Catfish warn that it gorges itself and that overfeeding comes very easily. OATA advises feeding only what the fish eats within a few minutes, once a day; for this fish, err on the small side and watch that the body stays sleek rather than bloated. Feeding after the lights dim matches its night-time habits.

Acclimatise it and protect your hands

Seriously Fish warns that the whale catfish could inflict a nasty bite, so take care during maintenance. Keep hands out of the water at feeding time, and use a long siphon and tools. OATA advises care when handling any catfish because most carry spines on the pectoral fins; whale catfish pectoral spines are not serrated (Planet Catfish), but the bite is the real risk.

For arrival, OATA recommends the floating-bag method, or slow drip acclimatisation for sensitive fish, with the lights off and as little transport water as possible going into the tank. Our guide to acclimatising new fish explains both, and our delivery and Live Arrival Guarantee policy covers dispatch days and what to do if there is a problem.

Use a first-week checkpoint plan

• Day 1: lights off after release; no food.
• Days 2-3: a small piece of thawed mussel after dark; remove leftovers.
• Days 1-7: test ammonia and nitrite daily and watch that it swims steadily rather than gasping.

Watch for warning signs

Scaleless fish can react badly to some medications, so check any treatment is safe for catfish without scales and improve the water before treating.

Keep it away from other species

Seriously Fish advises keeping it alone, because it will eat smaller tank mates and often bites chunks out of larger fish too. Planet Catfish says it is not a fish to be trusted with others: its natural adult behaviour is biting lumps out of large river catfish such as the scaleless pimelodids, and Chris Ralph reports these whale catfish being found inside the body cavities of larger fish such as the Red-Tailed Catfish and Tiger Shovelnose. Younger aquarium fish seem less dangerous, perhaps because they are well fed, but they will always eat smaller fish. Test any idea in our tank mates tool.

Decide from the suits and avoid list

Even the armoured option is a risk, and Chris Ralph would keep no other fish with them. Browse more South American catfish and our rare and collector fish in the catfish and plecos range.

Understand its biology

Chris Ralph describes C. coecutiens as one of the largest of the so-called candiru, a name used loosely for several small South American catfish that feed on other fish. It is not the tiny parasitic candiru of popular stories, but in the wild it is an opportunist that bites flesh from other fish and scavenges carcasses, and it is found in large numbers. FishBase notes that whale catfishes have an extremely small swim bladder enclosed in a bony capsule.

Breeding has never been achieved in the hobby (Seriously Fish). Mature females are fuller-bodied, and males are slimmer with a dorsal fin that tapers to a more pointed tip (Seriously Fish, Planet Catfish). FishBase lists both C. coecutiens and C. orinoco as Least Concern on the IUCN Red List.

Answer Whale Catfish buying questions

What is the Whale Catfish sold as Cetopsis sp. 'Orinoco'?

It is a whale catfish (family Cetopsidae) exported from the Orinoco region of South America under a trade label rather than a confirmed species name. Whale catfishes are smooth, scaleless, torpedo-shaped catfish with small eyes; the best known, Cetopsis coecutiens, scavenges and bites pieces from other fish in the wild.

How big will it grow?

That depends on the species, which is not confirmed. The large Cetopsis coecutiens, which lives in the Orinoco, reaches 26.5 cm standard length (Seriously Fish, Planet Catfish), while the described Cetopsis orinoco reaches only 9.4 cm (FishBase). Plan for the larger figure.

Will a Whale Catfish attack my other fish?

Yes, very likely. Seriously Fish says Cetopsis coecutiens eats smaller tank mates and often bites chunks out of larger fish, and in nature it rips flesh from fish much bigger than itself. Keep it in a species tank.

What size tank does it need?

For a fish that may reach 26.5 cm, Seriously Fish gives a minimum of 120 × 45 × 45 cm (about 243 litres). Chris Ralph, writing for Planet Catfish and ScotCat, suggests 72 × 24 × 24 inches (about 183 × 61 × 61 cm) for adults. Both stress strong flow and dim light.

What water does it need?

Seriously Fish gives 22-26°C, pH 6.0-7.4 and 2-15 dGH; Planet Catfish 22-26°C and pH 6.0-7.4; FishBase and ScotCat allow up to 28°C. Aim for 22-26°C and pH 6.0-7.4 with strong current and plenty of oxygen.

What does a Whale Catfish eat?

Meaty foods: prawns, mussel, cockle, lancefish and earthworms, with chopped mussel and live earthworms as favourites. Some accept shrimp-based tablets or dried foods. Do not use feeder fish, and feed carefully because it gorges itself.

Can I keep more than one?

Possibly. In the wild these fish feed in groups, and Seriously Fish reports some success with groups because they seem less likely to treat their own kind as food. Chris Ralph suggests two or three in a species-only tank, but Seriously Fish still advises keeping one alone as the safe default.

Is it the candiru that attacks people?

It belongs to the whale catfishes, a different family from the tiny parasitic candiru of popular stories, though Planet Catfish calls Cetopsis coecutiens one of the largest of the so-called candiru. It can bite, so keep hands clear during maintenance.

Consult the care references

• Seriously Fish: Cetopsis coecutiens (Blue Whale Catfish): distribution, habitat, size, aquarium size, water values, diet, behaviour and sexing of the blue whale catfish.
• FishBase: Cetopsis coecutiens: classification, maximum length, temperature, distribution and IUCN status of Cetopsis coecutiens.
• Planet Catfish: Cetopsis coecutiens: common names, sexing, water values, feeding, furniture and compatibility of Cetopsis coecutiens.
• Planet Catfish Catfish of the Month, August 2009: Cetopsis coecutiens: field sizes, body shape, eyes, colour, tank size, lighting, water changes and group keeping.
• ScotCat: Cetopsis coecutiens factsheet: water values, tank size and dim lighting for Cetopsis coecutiens.
• FishBase: Cetopsis orinoco: size and distribution of the small, described Cetopsis orinoco.
• Planet Catfish: Cetopsis orinoco: type locality, size and synonyms of Cetopsis orinoco.
• FishBase: family Cetopsidae (whale catfishes): family features of the whale catfishes: naked body, three pairs of barbels, no adipose fin.
• OATA: How to look after freshwater catfish: UK guidance for catfish on water quality, sand, introduction, handling, feeding and health signs.
• OATA: How to look after very large fish / tankbusters: UK guidance for very large fish on aquarium size, filtration, cover, introduction and feeding.
• Tropical Fish Co delivery policy: current dispatch days and Live Arrival Guarantee terms.