Wels Catfish (Silurus glanis)

What is a Wels Catfish?

The wels catfish (Silurus glanis) is the largest purely freshwater fish native to Europe. Wild specimens have been reliably measured at over 2 m (6.6 ft) and 130 kg (287 lb), and the species is documented to live 20–60 years. Its silhouette is unmistakable: a broad, flattened head with an enormous terminal mouth, tiny vestigial eyes, and six sensory barbels — two long maxillary barbels and four shorter chin barbels used to detect vibration and chemical signals.

The body is scaleless, smooth and heavily muscled, tapering to a long, paddle-like tail. Colouration is cryptic: dark olive-green to grey-black on the back, blending to pale mottled flanks and a cream-white belly. In the aquarium hobby the wels exists in a specialist niche — kept by experienced “monster fish” keepers as a long-term specimen, it demands extraordinary infrastructure and is unsuitable for any community setting.

Where do Wels Catfish come from?

The wels catfish is native to a vast swath of Europe and western Asia, from the Rhine and Elbe drainages in the west through the Danube basin and eastward to the Caspian and Aral Sea drainages. It favours large, slow-moving rivers, deep lakes and reservoirs where it finds dense cover — submerged roots, undercut banks and silt-filled pits. This is a temperate species; cold winters and mild summers in its native range explain both its cool preferred water temperature and its tolerance of near-freezing conditions.

The species has been introduced across western Europe (UK, France, Spain, Italy) where it is now considered invasive. In aquaculture it is farmed commercially for food. Wild-caught specimens entering the hobby are rare; nearly all trade fish originate from aquaculture or specialist breeders.

What size tank does a Wels Catfish need?

The 20,000 L (5,300 US gal) minimum reflects responsible housing for a sub-adult wels — a fully grown adult requires an institutional indoor pond or purpose-built outdoor system. Growth is rapid: a juvenile of 10–15 cm (4–6 in) can reach 50 cm (20 in) within its first year and 1 m (39 in) within two to three years.

A 500–1,000 L (130–265 gal) tank accommodates a juvenile for perhaps six to twelve months, not years. Any enclosure needs a tight-fitting, weighted lid — wels displace loose covers — plus at least one large cave or PVC pipe hide. Keep decor minimal and heavy; the species will uproot or break anything not firmly anchored. Fine sand or bare-bottom is easiest to maintain.

What water parameters do Wels Catfish need?

• Temperature: 15–25 °C (59–77 °F). This is a temperate fish; sustained temperatures above 26 °C stress it and reduce dissolved oxygen. It can tolerate brief dips toward 10 °C (50 °F) but should not be kept cold long-term.
• pH: 6.5–7.5, soft to moderately hard.
• Hardness: 5–15 dGH.

Stability is more critical than precision. Large systems with heavy bioloads are vulnerable to ammonia and nitrite spikes, so oversized biological filtration — typically sump-based with substantial media volume — is non-negotiable. Weekly partial water changes of 20–30% are the minimum for water quality management. A reliable thermometer and regular ammonia/nitrite/nitrate testing are essential husbandry tools given the fish’s sensitivity to chronic poor water quality.

What do Wels Catfish eat?

The wels catfish is a strict carnivore and opportunistic ambush hunter — wild fish take prey ranging from smaller fish and frogs to crayfish and small mammals. Overfeeding is the leading cause of water-quality problems in captive systems, so portion discipline matters.

Juveniles (under 30 cm / 12 in) accept large earthworms, defrosted prawns, lance-fish and whitebait. As the fish grows, whole thawed fish — trout, smelt or sprats — become the staple and mirror the natural diet. Some individuals can be trained onto large carnivore or sturgeon pellets, which reduce waste. Feed juveniles two to three times per week; larger specimens once a week or less. Remove uneaten food promptly and avoid feeder goldfish, which carry disease risk and offer poor nutrition.

Are Wels Catfish aggressive — and what fish can live with them?

Bluntly: the wels catfish is aggressive by nature and a capable predator of anything it can fit in its mouth — which, at adult size, is very nearly anything. It is not a community fish under any circumstances. Even juveniles will consume tank-mates that appear to be far too large to be at risk; the wels mouth opens to a remarkable gape and the fish is a powerful suction feeder.

With other wels catfish of similar size, uneasy cohabitation is sometimes possible in very large systems with ample feeding and space, but aggression, territorial biting and competition for hides can cause serious injury. The species is best housed as a solitary specimen, with min_group_size of 1 reflecting this reality.

For keepers interested in large-predator compatible species, some monster-fish specialists house wels with large, robust sturgeon of comparable size in very large pond systems — sturgeons’ armoured scutes offer some protection — but this remains a specialist undertaking with significant risk. For detailed compatibility information, see Wels Catfish tank mates.

How do you tell male and female Wels Catfish apart?

Outside of spawning season, sexing wels catfish by external appearance is difficult in juveniles and sub-adults. In mature fish, females grow larger and develop a noticeably deeper, rounder body profile as they fill with eggs — the most reliable visible indicator. Males during breeding season develop a slightly swollen urogenital papilla, which requires close examination and is most apparent when the fish is calm or sedated for inspection. In practice, most keepers maintaining a breeding pair confirm sex through size differential in fish of the same age cohort: the larger, deeper-bodied individual is almost always female.

How do Wels Catfish breed?

Breeding the wels catfish is rated very hard and has been achieved primarily by aquaculture facilities and a handful of specialist private keepers with large outdoor pond systems. Spawning occurs in late spring to early summer when water temperatures rise into the 18–22 °C (64–72 °F) range after a cold winter period — a seasonal trigger that is difficult to replicate in a constant-temperature indoor system.

The male guards a shallow nest among aquatic vegetation; after the female deposits adhesive eggs he fertilises them and tends the nest through incubation of roughly three to ten days. Fry are relatively large at hatching but require high-volume live food (earthworms, small feeder organisms) from the outset. Commercial aquaculture routinely uses hormonal induction, which is not accessible to private keepers.

What are common Wels Catfish diseases?

The wels catfish is robust with few species-specific diseases, but its heavy bioload makes water-quality-related illness the primary concern.

Columnaris and bacterial ulcers develop from poor water quality, physical injury (common in undersized enclosures), or stress. Anchor worm (Lernaea) and fish lice (Argulus) are risks for pond-sourced fish or those fed wild-caught prey. Fungal infections follow skin damage or deteriorating water. Intestinal parasites are common in wild-caught specimens.

Prevention rests on three pillars: oversized biological filtration with consistent water changes, adequate space to prevent self-injury, and a four-to-six-week quarantine for any new specimens before introduction to an established system.

Health note: disease diagnosis and medication dosing are beyond the scope of a care profile. Before treating a sick fish, confirm the diagnosis against a reputable veterinary or fish-health source — misidentification and incorrect treatment can be as harmful as the disease itself.

How long do Wels Catfish live?

A well-kept wels catfish lives 20–60 years — one of the most extraordinary lifespans in the freshwater hobby. A juvenile purchased today may outlive a decade of dedicated care and multiple home moves. Public aquaria that accept re-homed wels report individuals living well into their third decade.

Before acquiring any wels, answer the lifespan question honestly: where will this fish live in five years, in twenty? Responsible keepers agree an exit plan — a public aquarium, specialist facility or large private pond — before the fish comes home.