Iridescent Shark

The iridescent shark (Pangasianodon hypophthalmus), also sold as the Siamese shark, Sutchi catfish or Pangasius, is a large shark catfish native to the Mekong, Chao Phraya and Maeklong basins of Thailand, Laos, Cambodia and Vietnam. Wild stocks are IUCN Endangered, but it is one of Asia's most important aquaculture species, which is why thousands of juveniles reach the aquarium trade every year - usually sold as 5-8 cm silvery-blue youngsters with almost no warning of what follows. It is a pelagic, open-water river fish that can live 20 years or more.

Wild fish reach 130 cm and 44 kg; in captivity they rarely hit 130 cm but easily top 90 cm, so the profile's 100 cm working figure is reasonable while still leaving no margin. They can live 20 years or more, making this one of the longest commitments in the hobby. The profile's 100 cm must be planned for from the day a 5 cm juvenile is bought, because growth is relentless and the tank does not slow it - undersized housing produces a panicked, injured fish, not a small one.

Iridescent sharks are peaceful but nervous open-water swimmers with poor eyesight: startled, they dash chaotically and injure themselves on glass, decor and tankmates, which is why crumpled tanks kill them. They are not aggressive - they are easily intimidated by boisterous fish and will eat anything small enough to swallow - and they constantly cruise the middle water column, bothering sedentary tankmates simply by being busy. Young fish school; adults become more solitary but still want their own kind, so plan for company and space together.

Adult iridescent sharks need 450 x 180 x 180 cm - about 14,580 litres - as a long-term minimum for even one fish (Seriously Fish), with FishBase's >150 cm and Aquario's 2,000 L as the more permissive readings; the profile's existing 1,000 L fails all three. Fit a suitably sized, heavy cover, because it occasionally breaches the surface during bouts of nervous behaviour, and secure every heater, thermometer and cable because a 100 cm+ specimen destroys equipment. Keep the tank dimly lit with low, secured hardscape, strong biological filtration and no narrow gaps where a panicked fish can wedge or injure itself.

Read the minimum as floor area first: 450 x 180 cm of usable base - a long, wide box - because this is a pelagic open-water swimmer that turns at the ends of a straight lane. A tall tank can match the litres while giving a 100-130 cm fish no room to bank, and a skittish fish in a tight space bangs into glass and decor and injures itself. Keep the central lane clear and secure anything it can hit at speed.

Omnivore with an age-based shift: juveniles need a protein-rich diet (chopped earthworm, bloodworm, pellets) to develop correctly, while adults need a growing share of vegetable matter - spinach, peas, Spirulina foods and occasional fruit. In the wild they take fish, crustaceans and vegetable debris, and in old age they lose teeth and lean further toward vegetation. Feed small portions two to three times a day that are finished within 5-10 minutes, and do not let a greedy fish drive overfeeding - waste management is already the main maintenance load.

Not aggressive, but three ways to fail: anything small enough to swallow is eaten, sedentary or shy fish are stressed by constant cruising, and aggressive or boisterous fish intimidate it into panicked, self-injury dashes. In genuinely huge systems the candidates are large, peaceful benthic and shoaling fish such as doradids, pacu or large cyprinids - all far too big to swallow. For most keepers that math does not work, which is why FishBase simply says not recommended for home aquariums: in practice this is a species-only fish.

Verdict: one of the most consistently mis-sold fish in the hobby - do not buy it unless you already have pond or public-aquarium scale facilities. The published long-term minimum is 450 x 180 cm (about 14,580 litres) for a single fish, and it wants a group of five; a 1,000 L tank fails by more than an order of magnitude however good the filtration. Budget 20 years of heavy filtration, secured equipment, a heavy lid, open swimming space and a nervous 100-130 cm fish that injures itself when cramped - and remember that public aquaria are already inundated with cast-offs they cannot take.