Iridescent shark

The iridescent shark (Pangasianodon hypophthalmus) is the most consistently mis-sold fish in the freshwater hobby. A 3-inch juvenile in a chain-store display tank looks like a graceful, silvery community fish — sleek body, tall dorsal fin, almost metallic sheen along the flanks. What that display tag rarely mentions is that the same fish, given competent care, will grow into a 4-foot riverine catfish that needs more swimming room than most people have in their living room.

This is not a shark. It is a giant catfish from the Mekong River, sold under at least four trade names — iridescent shark, Pangasius catfish, Siamese shark, and (in its leucistic form) albino iridescent shark. The "shark" branding is pure marketing built on a passing resemblance to the body shape of a true shark. The biology is closer to a freshwater tuna: open-water swimmer, powerful musculature, schooling instinct, and a metabolism built for moving water and abundant food.